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Electronics => Metrology => Topic started by: zlymex on May 11, 2016, 11:43:07 pm

Title: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on May 11, 2016, 11:43:07 pm
Requirement
 - able to test low frequency noise of voltage references
 - 0.1Hz to 10Hz bandwidth
 - portable, easy to use
 - low floor noise, 0.16uVpp level(same as Linear described in AN124f by Linear)

General considerations
 - self-contained, not necessary depending on oscilloscope or DMM for reading
 - one 9V rechargeable lithium battery powered
 - charge port, oscilloscope output port provided
 - case: aluminium, 103mm by 76mm by 35mm, my usual type
 - LED display: 4.5 digits

Schematics
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=224688;image)
There are several modifications to the Linear one
1. Input capacitor C1
I bought those wet tantalum several years ago but tested not good recently. After applied 10V for 10 hrs, there is still leakage of several uA. May be the voltage I applied is much larger than Linear(2.5V) or simply the caps are bad. Either way, I give up. I have several 80uF and 22uF film caps, tested very good, but requires too many  to built up to 1000uF, and the volume is huge. Now the only option left seems to be MLCC. I bought some of those 47uF/50V before and there are still about 50 left. However, there are problems when I installed those capacitors, will be described later.
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=224113;image)

Add: later test showed that the current noise of ADA4528 is much smaller than specified. Therefore, a smaller value for C1 can be used such as 1000uF or 470uF.

2. Input resistor R1
It provides DC bypass for C1 to charge/discharge, and also function as the 0.1Hz HPF together with C1. The attenuation is -1.5dB at 0.1Hz because there is another HPF at later stage with similar property, summing up to -3.0dB at 0.1Hz.

3. Input protection
I first implement it without Rp2, D3 and D4, an opamp was fried.
Because the noise current pass thru those Rp1 and Rp2, keep them as low as possible, better not exceed R1 combined.
I use bc junction of low power transistors(such as 2SC1815, 2SA9012) for D1-D4, leakage current is around 1pA at -4V.
R2 and the switch provide slow charge for the capacitor when connect to a voltage source.

4. The amplifier
The magnification is 10000, same as Linear.
I don't want to use FET front end, and I'm not prepare to measure >10Hz.
My target is a dual opamp, voltage noise <=100nVpp  0.1Hz to 10Hz. There are many so called ultra low noise opamps that not satisfied this because they suffered from severe 1/f noise effect. For instance ADA4898 with 500nVpp noise, even if 20 paralleled, still >100nVpp.

Also, the current noise should be <=50pApp in 0.1Hz to 10Hz range, which generates <=50nVpp voltage noise at 1k impedance. Be noted also that there are many so called ultra low noise opamps that not satisfied this.

It seems that there are not many opamps left to satisfy these criteria except ADA4522-2, but I cannot find the source of purchase. I choose  ADA4528-2 instead with very similar performance except the supply voltage is a bit low.

5. Post opamp part
I use two amplifiers in parallel to further reduce the noise, the outputs are connected together by two 620 ohm resistors, and add an 33uF capacitor(C3) for 10Hz LPF. Now the LPF is second order.
Becasue C4 and R5 is another 0.1Hz HPF, there is no need for separate filter stage as in AN124f circuit.

6. Meter part
This part can be omitted if one decide to use an oscilloscope only for output. 
Unlike Liner that use paralleled bc junction and be junction of transistors for the low leakage diodes, I use only bc junction for the peak detection diode. The reverse break down voltage of a be junction may be very low, therefore Linear has to use added resistors and diodes for clamp.
R6a and R6b are just jumpers, value not important.
U4a and U4b are photMos AQY212GS, leakage only around 1pA at -4V and turn on current of only 1.5mA with sub-ohm turn on resistance.
(There is no photoMos symbol in the software I use, so that I use phototransistor symbol insdead)
U2B is the instrument amp because my LED meter is earth referenced.
I use manual reset switch only, although an automatic reset can be added if required.
The LED meter has 1.9999V range, operable from 3.4V to 20V and draw 18mA current. I modified the decimal point so that it display 199.99(uV)

7. Power supply
There are two lithium cells inside a 9V rechargeable battery, supply 7.2V to 8.4V for small current, nominal 8V.
U2A split this 8V to +4V and -4V. I choose RS3 a bit small so that it share the current necessary for LED meter which only draw current from positive rail.
D5 and D6 provide the negative supply for U1(max 5.5V for this ADA4528-2). This can be omitted if U1 is an ADA4522-2.
D7 and D9 provide charge route by input socket when power off. Caution should be taken to disconnect anything from the input socket when power off.
Rp3 provide the trickle charge current for C1 when power is off.

Here is the photo of the finished meter:
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=224115;image)
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 11, 2016, 11:45:17 pm
Building process, modifications and lesson learned

Firstly, get together all the large parts such as the case, board, LED meter, battery and sockets, arrange them till satisfied. I'm not going to make a PCB since this is a test build, modification is inevitable.
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=224123;image)

Sockets are to be used for two opamps for easy replacement owning to test or damage. However, soldering of those MSOP is not an easy task for me.
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=224125;image)

Secondly, assembling. It almost out of hand since there the modification is continues and I'm running out of board space. I could have move the U1 toward left side but hindered by the centered LED. Anyway, here is the inside photo of workable first version.
(http://www.eevblog.com/forum/index.php?action=dlattach;topic=67916.0;attach=224128;image)

I only installed 21 MLCC caps(the plan is 36, 1500uF), tested not good at all. It must be the severe piezoelectric effect of the MLCC that I greatly under estimated. I cannot touch or move the meter while the measurement is in progress. The touching of the table or even waking nearby seems affect the result. Here is the pulse I got when I slightly press the reset button.
(http://www.eevblog.com/forum/index.php?action=dlattach;topic=67916.0;attach=224130;image)

Even if everything is idling, there is still exit low frequency wobbling probably because the very large TempCo or stress release or something.
(http://www.eevblog.com/forum/index.php?action=dlattach;topic=67916.0;attach=224132;image)

Long story short, after I've tested many aluminum electrolytic capacitors, I found several very good one for the job, very low leakage at 10V(<15nA) and there is no aperiodic noise bursts at all mentioned by Linear. They are:
one Nippon Chemi-Con 1000uF 35V, one Nippon Chemi-Con 2200uF 35V, two Panasonic 3200uF 35V

I use 2200uF 35V as the final selection, here is the inside photo.
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=224133;image)

I'm very happy with this since the capacitor settle down very quickly, allowing me to switch references with no time especially for similar voltages.
As a comparison, it took Linear 24 hours to settle down their highest grade $400 price tag wet slug.

More modification on 14th May: Change value of R1, R2a, R2b, R3, R5 and C3 so that the frequency band is precise.
Also, added Rp3 and D7 for protection and R2 for slow charge of C1. After the modification, the floor noise of the meter is slightly increased from 90nVpp to 100nVpp, still well within the original planned 160nVpp.
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=225191;image)

Noise floor of my oscilloscope PicoScope6, input shorted by a special BNC cap, only 7.5nVpp, thanks Andeas proving a very useful software probe.


A quick way to settle down the input capacitor.
EE caps have very large DA and an 2200uF can be roughly models as:
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=225703;image)

When I connect a DUT to the meter, sometimes it takes more than 15 minutes for C1 to settle down so that U1 get out of saturation. The direction of the 'leakage' current has both ways: it can flow into the R1 or flow out of it depending on the history/present voltage of the capacitor. There is a quick way though for C1 to settle down fast: 'reverse' bias it. For instance, if C1 stayed at 8V for a long time(as happened when the power is turn off for long time and just turned on), I need to test 6.3V, then I'll short the input to ground for 3 seconds so that the voltage of C1 is about 1V, then I wait for one minute before actually connect to the DUT. The C1 will now settle down much quicker. However, there are times that the waiting is still too long afterwards, and I don't know which direction the leakage of the C1(the knowledge of the direction is important so that I can repeat the revere process). I solve this by adding two LEDs as can be seen below to  indicate the saturation status so that further steps can be taken.

Simple version of the noise meter
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=233006;image)
Major modification: omit the sample-hold and LED display, use one lithium cell and ADA4528-2, use smaller input capacitors since the actual current noise is very small.
The floor noise is 90nVpp(0.015uV RMS).

Operation Procedures
(Take 7V reference noise measurement for example)
1. Prepare the DUT and the noise meter(check for battery, charge if necessary)
2. Disconnect anything from the BNC input of the noise meter, connect to oscilloscope, turn the meter on, both LEDs should be lit.
3. Connect the cable to DUT(no connection to the meter yet), measure the voltage from the BNC plug to confirm the test voltage
4. Measure the voltage of C1 from the BNC input socket of the meter, should be around 8V(same as battery voltage).
5. Short the BNC socket of the meter for 3 seconds. This will make the voltage of C1 drop to around 1V.
6. Wait for 1 to 2 minutes so that C1 is 'reverse' exercised
7. Connect DUT, now the blue LED should be out indicating the negative saturation of the U1.
8. Wait for 2 to 3 minutes, the blue LED should be come back on. If the green LED goes out quickly, C1 is not exercised enough, goto step 5
9. If the waiting is too long(>5 minutes), then the exercise is excessive. Disconnect the BNC plug from the meter, turn off the meter(so that C1 is charged) for 3 seconds and turn on again, plug the BNC back. This step can be repeated.
10. If both LEDs are on for sometimes, the measurement can be performed.
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 11, 2016, 11:49:28 pm
Some measurement result
Note also that the results here is only the noises that I tested using my DIY meter on particular voltage references that I possess.

1. Panasonic 3200uF/35V capacitor(I was told this is used for airbags), charge to 4.1V, 124nVpp
2. Panasonic NCR18650B lithium battery, charged to full more than six months ago, 4.1V, 115nVpp
3. A Chinese temperature compensated 6.3V zener, 2DW233, powered by 12V battery thru 1k resistor(5.7mA), 336nVpp, much better than a LTZ1000.
This ultra low noise characteristic of the 2DW23x series(from a particular maker) has been confirmed by many Chinese voltnuts before, but I don't believe this until I had my own test.
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=224382;image)
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=224466;image)
When current increased to 11.8mA, noise is reduce even further to 236nVpp.

4. Other measurement result is summarize in table below
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=225180;image)


5. Ordered by noise
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=225182;image)

6. Some words about 2DW23x
The one I tested is Diamond brand made by Shanghai 17th Radio Factory. I have a lot of other 2DW23x which are much inferior with noise figures ranging from 20uVpp to 100uVpp. The design and construction of this 2DW23x were completely changed although they still share the same datasheet.

Understandably the noise of a zener is inverse proportional to the square root of the zener current in theory,  and in practice I tested that 2DW233 follows this very well. The mystery is, how they achieve this kind of low noise?
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=225203;image)

I took apart one and took a photo with my card camera plus a magnifier. It seems to me that they are hand made because the die is not centered and wires are irregular.
Title: Re: DIY low frenquency noise meter
Post by: Andreas on May 12, 2016, 01:21:05 am
I first implement it without Rp2, D3 and D4, an opamp was fried.

Shure it was through the input?
The AD4528-2 is specified for 5.5V single supply and +/-2.75V dual supply.

Nice scope. Which PicoScope Model do you use?

I bought those wet tantalum several years ago but tested not good recently. After apply 10V for 10 hrs,

10 hrs may be not enough.
At the moment I try forming the input capacitor from 9.5 (my 9V block which is usually attached to the input) to 13V because I recognized large noise at above 10V. After 4 days now the noise level is settling from several uVpp (> 4uV) to now around 250nVpp. So you should keep the input capacitor constantly charged somewhat above the voltage that you want to measure.

With best regards

Andreas
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 12, 2016, 02:15:52 am
Hi Andreas,

About the burn down of the opamp, it happened very soon, I only have chances to test 2 or 3 voltages. I don't know exactly how, but it's Ok after I added Rp2, D1 and D2, there is no problem since. The normal supply for U1 is 5.2V, not 8V.

The PicoScope I use is 5442A. The one Dave's teardown(EEVBLOG #521) is 5443B

For those wet tantalum, I'm giving up. I cannot wait that long for a result because my intention is universally quick test, and the voltages may be different from time to time. My capacitor is constantly charged if switched off as can be seen from the schematics.
Title: Re: DIY low frenquency noise meter
Post by: Kleinstein on May 12, 2016, 03:02:32 am
For charging the input cap to 8 V when not active one could add a resistor to prevent bad things happen when connecting something to the input in this case.

It might also be a good idea to have a switchable series resistor at the input to prevent excessive load to the DUT. Some reference circuits don't like such a current spike.

Are the Phototransitstors good enough to discharge the peak detecting caps far enough - they might have a saturation voltage in the 50 mV range.
Title: Re: DIY low frenquency noise meter
Post by: Vgkid on May 12, 2016, 03:07:16 am
Thanks for the circuit. Do you have any information on those 2dw233 zener diodes?
Title: Re: DIY low frenquency noise meter
Post by: SeanB on May 12, 2016, 05:10:43 am
With those big rectangular Tantalums I will tell you the bad news. Internally they consist of 2 PCB assemblies, with regular rubber bung tantalum slug capacitor units soldered between 2 pcb end pieces, and then with PTFE wire leads to the lid section. Then this is slipped into a kapton sleeve and dropped into the can, which is then filled with a white silicone sealer, pretty soft and flexible. The lid is then put on after curing and soldered.

They are fiendishly expensive from Kemet now, and are sadly the one product they make in glass frit seal tantalum that is pure shyte, the proper glass frit seal capacitors are good essentially forever, or till the case corrodes through from the outside. The rubber bung style capacitor is used because the end cap is not soldered, so will withstand a solder bath immersion at least once. Sadly the seal is not gas tight, and with time the wet sulphuric acid electrolyte inside will diffuse through the rubber bung, and eventually the capacitor goes high ESR , and is considered failed, or leaks sulphuric acid.

Your ones having high leakage says they have dumped acid internally, and have this sitting inside the case giving the high leakage. Time to make yourself a small set of beautiful coloured ( the anode slugs are a lovely blue green colour, depending on the formation voltage and the degradation from storage) figurines in some stoppered glass test tubes, just by keeping them in there in some dilute sulphuric acid.

I was working on equipment that used these, and did rebuild a few using regular electrolytics inside the cases, as the mounting was designed to only fit these style units. I think I changed around 5000 dead tantalums over the course of a year, running through the pile of boards.
Title: Re: DIY low frenquency noise meter
Post by: Marco on May 12, 2016, 06:32:28 am
I don't know anything about the zener, but I was intrigued so I looked up the datasheet (http://www.chinaeds.com/zl/%E5%B8%B8%E7%94%A8IC%5C2%5C2D%5C2DW230.pdf). Seems a simple planar zener, the third pin is superfluous.
Title: Re: DIY low frenquency noise meter
Post by: Andreas on May 12, 2016, 07:16:18 am
The PicoScope I use is 5442A. The one Dave's teardown(EEVBLOG #521) is 5443B

Hello zlymex,

there is still a lot of wideband noise (above 10Hz from the scope) in your screenshots. (i.e. the 73 nV)
I would either set the resolution to 16 Bits (if you use only 1 channel) and 20 Bits resolution enhancement. (in the channel menue).
Or: set the resolution to 16 Bits  and use a 1kHz or 100 Hz digital filter.
Maybe you have to increase the number of samples from 1 Meg to 5 Meg.
So you should get a nearly clean dash for the scope.

Edit: note the picture is with 50x magnifier.

With best regards

Andreas
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 12, 2016, 10:13:45 am
For charging the input cap to 8 V when not active one could add a resistor to prevent bad things happen when connecting something to the input in this case.

It might also be a good idea to have a switchable series resistor at the input to prevent excessive load to the DUT. Some reference circuits don't like such a current spike.

Are the Phototransitstors good enough to discharge the peak detecting caps far enough - they might have a saturation voltage in the 50 mV range.
Thanks very much, good points made for the first two, I'll modified the schematics and the circuit.

For the third one, I forgot to mention that I actually using photoMos in the circuit. There are no photoMos symbols in the drawing software(Multisim) so that I use phototransistors in the schematics instead.
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 12, 2016, 11:19:51 am
Thanks for the circuit. Do you have any information on those 2dw233 zener diodes?
Those 2DW23x series(2DW232, 2DW233, 2DW234, 2DW235) has been around for many year and are the only 'reference grade' zener in China. Previously known as 2DW7C and changed name about ten years ago. There are a lot of manufacturers making these devices and I have tons of them. The performance varies according to makers very much and generally are very poor (aging, tempco, noise). I've never use them in my designs/circuits.

However, someone discovered that there is this Shanghai 17th Radio Factory making this particular Diamond brand(there is a diamond symbol on face of each device) with exceptional low noise. I didn't believe it first, but people start buying/teardown/discuss about it since and more evidence for the low noise emerged.

Those 2DW233 I bought is from here: https://item.taobao.com/item.htm?spm=a1z09.2.0.0.gpf2Zk&id=35815633601 (https://item.taobao.com/item.htm?spm=a1z09.2.0.0.gpf2Zk&id=35815633601)
Noise comparison tests
 http://bbs.38hot.net/forum.php?mod=viewthread&tid=49306 (http://bbs.38hot.net/forum.php?mod=viewthread&tid=49306)
http://bbs.38hot.net/forum.php?mod=viewthread&tid=84620 (http://bbs.38hot.net/forum.php?mod=viewthread&tid=84620)
http://bbs.38hot.net/forum.php?mod=viewthread&tid=119921 (http://bbs.38hot.net/forum.php?mod=viewthread&tid=119921)
http://bbs.38hot.net/forum.php?mod=viewthread&tid=120264 (http://bbs.38hot.net/forum.php?mod=viewthread&tid=120264)
Teardown and analysis http://bbs.38hot.net/forum.php?mod=viewthread&tid=120731 (http://bbs.38hot.net/forum.php?mod=viewthread&tid=120731)

There are many sellers at Aliexpress selling these cheaply but only buy those with diamond mark on the top such as
http://www.aliexpress.com/item/Free-shipping-2DW233-DIP3/32433963421.html (http://www.aliexpress.com/item/Free-shipping-2DW233-DIP3/32433963421.html)
http://www.aliexpress.com/item/hot-spot-10pcs-2DW232-new-original-in-stock/32637084815.html (http://www.aliexpress.com/item/hot-spot-10pcs-2DW232-new-original-in-stock/32637084815.html)
And preferably recently made. The first two digits on the bottom are year of make, mines are 14 and 15(2014, 2015)

The difference within the series is only the zero tempco current. For 2DW232, its 5mA. 2DW233 is 7.5mA. 2DW234 is 10mA. I prefer 2DW232 and 2DW233, but it seems not much difference, most of those actual zero TC points are larger than specified ranging from 7mA to 20mA.

I'm not giving much hope to these devices because the aging rate is a question mark.
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 12, 2016, 12:02:53 pm
......
Your ones having high leakage says they have dumped acid internally, and have this sitting inside the case giving the high leakage. Time to make .......
I've just tore apart one, there are 5 smaller caps inside.
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 12, 2016, 12:44:30 pm
I don't know anything about the zener, but I was intrigued so I looked up the datasheet (http://www.chinaeds.com/zl/%E5%B8%B8%E7%94%A8IC%5C2%5C2D%5C2DW230.pdf). Seems a simple planar zener, the third pin is superfluous.
That is correct. There are two zeners in symmetrical back to back connection, therefore we can use it in either ways. The third pin is the common cathode and is connect to the case. It used to be planar structure but I'm not quite sure for these Diamond brand.

Edit: I took apart one, here is the photo by my card camera plus a magnifier
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 12, 2016, 02:09:56 pm
The PicoScope I use is 5442A. The one Dave's teardown(EEVBLOG #521) is 5443B

Hello zlymex,

there is still a lot of wideband noise (above 10Hz from the scope) in your screenshots. (i.e. the 73 nV)
I would either set the resolution to 16 Bits (if you use only 1 channel) and 20 Bits resolution enhancement. (in the channel menue).
Or: set the resolution to 16 Bits  and use a 1kHz or 100 Hz digital filter.
Maybe you have to increase the number of samples from 1 Meg to 5 Meg.
So you should get a nearly clean dash for the scope.

Edit: note the picture is with 50x magnifier.

With best regards

Andreas
Thanks very much for the suggestion. Yes, there was lot of wideband noise that should not be appeared in the chart. I actually turn on the resolution enhancement(by default) but suspected the post-process so that I deliberately turn it off. I set it up according to your suggestion now I got around 20uVpp noise, much better than before. I then measured the noise floor of my meter, ranging from 83nVpp to 95nVpp, let's say it's 90nVpp.  I'lll update the charts above with blue remarks.
Title: Re: DIY low frenquency noise meter
Post by: Andreas on May 12, 2016, 03:31:22 pm
Hello Zlymex,

I have defined myself a custom specific probe with attenuation 0.0001:1 (amplification 10000 fold).

So I do not have to calculate the factor 10000 manually in my measurements.
(remove ".txt" from attachment before importing to picoscope software in the probe menu).

With best regards

Andreas
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 12, 2016, 06:10:42 pm
Hello Zlymex,

I have defined myself a custom specific probe with attenuation 0.0001:1 (amplification 10000 fold).
......
That's fantastic!
I imported it and now the noise floor of the PicoScope is only 7.5nVpp
I then measured that 2DW233 again @ 11.8mA, the noise is only 236nVpp and can be read directly from the peak to peak value at the bottom.
Title: Re: DIY low frenquency noise meter
Post by: Andreas on May 12, 2016, 08:08:09 pm
Hello Zlymex.

yes nVpp
you should also write it on the blue text within the picture and not uVpp!!
Otherwise someone could be confused.

By the way: how do you shield your DUT?
Cookies box or tin can?

With best regards

Andreas
Title: Re: DIY low frenquency noise meter
Post by: Alex Nikitin on May 12, 2016, 09:35:24 pm
Nice work, however I have to point out that the actual bandwidth of the circuit shown in the first post is somewhat smaller than required, only 0.16Hz to 6.5Hz (-3dB).

Cheers

Alex
Title: Re: DIY low frenquency noise meter
Post by: DuPe on May 12, 2016, 09:53:22 pm
Thanks zlymex for sharing this.
I like it, since it is more straightforward than big Jim's AN124.

Cheers
Peter
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 13, 2016, 01:09:05 am
Hello Zlymex.

yes nVpp
you should also write it on the blue text within the picture and not uVpp!!
Otherwise someone could be confused.

By the way: how do you shield your DUT?
Cookies box or tin can?

With best regards

Andreas
Oh yes, thanks, updated.
I just wrap tightly around the DUT with some soft tissue. Voltage references are all low impedance and they will not be easily affected if not put something like tin can. However, they are very sensitive to thermal changes so I have to make sure there is no circulation/wind for DUT especially on leads and connections.
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 13, 2016, 01:11:07 am
Nice work, however I have to point out that the actual bandwidth of the circuit shown in the first post is somewhat smaller than required, only 0.16Hz to 6.5Hz (-3dB).

Cheers

Alex
Thanks for that, can you tell me how the bandwidth of 0.16Hz to 6.5Hz is calculated?

Edit: Now I understand why. I've modified the schematics and my implementation so that the bandwith now is 0.1Hz to 10Hz(-3dB).
Title: Re: DIY low frenquency noise meter
Post by: Andreas on May 13, 2016, 02:10:43 am
Nice work, however I have to point out that the actual bandwidth of the circuit shown in the first post is somewhat smaller than required, only 0.16Hz to 6.5Hz (-3dB).

Cheers

Alex
Thanks for that, can you tell me how the bandwidth of 0.16Hz to 6.5Hz is calculated?

Hello,

Thats something that I have overlooked too:
In my design the input capacitor is somewhat away from the 0.1 Hz edge because of the large tolerances of the electrolytics.
(so I have 3200uF * 1K  = 0.05 Hz for the input and the lower edge frequency is determined by foil capacitors with 20uF).

for the 0.1 Hz edge:
input  1/(2100 uF x 750R * 2 * PI) = 0.101 Hz for the 1st -3dB point
Output 1/(50uF * 30K * 2 * PI) =  0.106 Hz for the 2nd -3dB point.
so both high passes add to -6dB at 0.1 Hz

The -3dB point is around factor sqrt(2) higher so around at 0.14 Hz.

I would do a LTSPICE simulation for the whole.

With best regards

Andreas




Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 13, 2016, 03:02:39 am
In that case, might probably increase the 750 to 910 Ohm, and increase 30k to 36k. And modify other resistors if necessary.
Title: Re: DIY low frenquency noise meter
Post by: Alex Nikitin on May 13, 2016, 05:54:57 am
In that case, might probably increase the 750 to 910 Ohm, and increase 30k to 36k. And modify other resistors if necessary.

As Andreas said, just do an LTSpice simulation of the circuit. That is what I did after I became suspicious of the frequency response with the values shown. 

Cheers

Alex
Title: Re: DIY low frenquency noise meter
Post by: Andreas on May 13, 2016, 06:22:39 am
Did a quick simulation of a equivalent filter:

(2*620 R in parallel = 310 R single)

with best regards

Andreas
Title: Re: DIY low frenquency noise meter
Post by: Kleinstein on May 13, 2016, 07:04:20 am
With noise measurements it's not the -3 dB point's that really count, but the integrated curve. The filter is not perfect and will still give some contribution from outside the -3 dB points. For white noise one uses a equivalent noise bandwidth. With the often dominating 1/f noise it gets more complicated. So to compare data one needs more than just the band limits, but also the type of filter, especially at the lower end.

Usually the RMS values are a little better to measure, as they don't fluctuate as much as the peak - peak values.
Title: Re: DIY low frenquency noise meter
Post by: Andreas on May 13, 2016, 07:13:05 am
And a possible dimensioning when you do not want to change the electrolytics.

With noise measurements it's not the -3 dB point's that really count, but the integrated curve.

Unfortunately there is no "standard" for measuring. Some use 2nd order other 4rth order cirquits.

The most important thing is: you have a cirquit where you can compare different references.
(And sort out the "stinkers").

@Zlymex: for the LM399 / LM329 the current should not play a large role above  0.5-1mA.
All current above around 250uA is shunted away from the zener element.

With best regards

Andreas
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 13, 2016, 10:57:46 am
I did the simulation before, I just took that -6dB fall off for granted. That's inevitable isn't it for a second order filter? I mean Linear did the same thing for their filters in AN124f where they choose 1300uF-1.2k and 165uF-10k for 0.1Hz HPF.

The question is, should I modify the parameters so that the fall off at 0.1Hz and 10Hz be -3dB?
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 13, 2016, 11:07:35 am
And a possible dimensioning when you do not want to change the electrolytics.

With noise measurements it's not the -3 dB point's that really count, but the integrated curve.

Unfortunately there is no "standard" for measuring. Some use 2nd order other 4rth order cirquits.

The most important thing is: you have a cirquit where you can compare different references.
(And sort out the "stinkers").

@Zlymex: for the LM399 / LM329 the current should not play a large role above  0.5-1mA.
All current above around 250uA is shunted away from the zener element.

With best regards

Andreas
Thanks very much for the simulation. I didn't see this second page until now because I've just got up in the morning and my mind is still in a half sleep stage. I'm sorry I'm unable to respond this thread in the next 8 hours time.

True for the zener current. I just got that 1k resistor and 12V battery at hand. That slightly larger current than required had some heating effect.
Title: Re: DIY low frenquency noise meter
Post by: Alex Nikitin on May 13, 2016, 07:24:26 pm
I did the simulation before, I just took that -6dB fall off for granted. That's inevitable isn't it for a second order filter? I mean Linear did the same thing for their filters in AN124f where they choose 1300uF-1.2k and 165uF-10k for 0.1Hz HPF.

So they've cheated  ;) .

The question is, should I modify the parameters so that the fall off at 0.1Hz and 10Hz be -3dB?

I suppose yes. Below is the simulation I've done to get -3dB points at right frequencies.

Cheers

Alex
Title: Re: DIY low frenquency noise meter
Post by: Andreas on May 13, 2016, 07:58:44 pm
I mean Linear did the same thing for their filters in AN124f where they choose 1300uF-1.2k and 165uF-10k for 0.1Hz HPF.

Ok good point: that explains (partly) why for the LTC6655 (5V) I get different results against the data sheet.
Datasheet value is 0.25ppmpp = 1.25uVpp.
whereas I got 2.2uVpp for the MSOP and 2.7uVpp for the LS8-package.

Of course LT will not change its cirquit so they would have to update their datasheets with higher 1/f noise levels.

The question is, should I modify the parameters so that the fall off at 0.1Hz and 10Hz be -3dB?

Its not necessary from my side. But in comparison with other amplifiers you will have perhaps -10 or -20% deviation.
TI obviously uses -3 dB corner frequency definition (at least for characterisation of OP-Amps).
But with a 2nd Order high pass and a 4rth order low pass.
http://www.ti.com/lit/ug/slau522/slau522.pdf (http://www.ti.com/lit/ug/slau522/slau522.pdf)
http://www.ti.com/tool/tipd122?keyMatch=0.1-10hz%20filter&tisearch=Search-EN-Everything (http://www.ti.com/tool/tipd122?keyMatch=0.1-10hz%20filter&tisearch=Search-EN-Everything)

So if you measure TI devices you will probably need a different amplifier ;-)  :-//

On the other side: Electrolytics have large tolerances (up to +80/-20%)
so for the lower limit (high pass) you will probably be already on the safe side.
But for the low pass 620R||620R + 2*100uF in series the upper 10Hz limit may be much too low if you do not have measured the capacitors before soldering in.

Edit: Alex was faster. (is the cirquit a "defeat device"?)

With best regards

Andreas
Title: Re: DIY low frenquency noise meter
Post by: alanambrose on May 14, 2016, 12:16:37 am
A very nice design and build - I think this wins the 'most functionality out of a smallish number of components competition' :)

Alan
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 14, 2016, 03:07:49 am
Thanks very much every body for the comments and suggestions especially for Andeas providing the probe and Alex providing the simulation.
Now I modified the schematics(uploaded in my first post) according to the parameters from Alex's which I've confirmed in my simulation.
I also modified my meter so that it reflect most of the changes in the schematics. It turns out that I've already use 1k for R1 owning to C1 in my first implementation is 1500uF. Now the floor noise of my meter is increase by about 11%. I will use purple letter for chart marking from now on.
Title: Re: DIY low frenquency noise meter
Post by: DuPe on May 14, 2016, 05:18:14 pm
Can someone help me to understand the virtual ground concept within this schematic?
Virtual ground is done by U2A. LMC6064 is able to provide 16mA max.
Assymetry introduced by the voltmeter (I took murata DMS-40PC series as example) is roughly 70mA for the low power version.
And RS3 only contributes ~20mA to the balance
How does this work?
Cheers
Peter
Title: Re: DIY low frenquency noise meter
Post by: zlymex on May 14, 2016, 08:49:31 pm
Can someone help me to understand the virtual ground concept within this schematic?
Virtual ground is done by U2A. LMC6064 is able to provide 16mA max.
Assymetry introduced by the voltmeter (I took murata DMS-40PC series as example) is roughly 70mA for the low power version.
And RS3 only contributes ~20mA to the balance
How does this work?
Cheers
Peter
My LED meter draws only 18mA current, therefore the U2A provides only about 2mA extra.
If that murata DMS-40PC has to be used here, a transistor booster to the U2A is necessary. However, I'm not quite sure about the loop stability.
Title: Re: DIY low frenquency noise meter
Post by: DuPe on May 14, 2016, 09:16:03 pm
Can someone help me to understand the virtual ground concept within this schematic?
Virtual ground is done by U2A. LMC6064 is able to provide 16mA max.
Assymetry introduced by the voltmeter (I took murata DMS-40PC series as example) is roughly 70mA for the low power version.
And RS3 only contributes ~20mA to the balance
How does this work?
Cheers
Peter
My LED meter draws only 18mA current, therefore the U2A provides only about 2mA extra.
If that murata DMS-40PC has to be used here, a transistor booster to the U2A is necessary. However, I'm not quite sure about the loop stability.
Thanks zlymex, this makes things fit together.

Cheers
Peter
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: splin on May 16, 2016, 04:02:56 am
Excellent work zlymex. It would be interesting to see the noise floor with a few values of R1, up to say 1M, to measure the actual input noise current.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on May 16, 2016, 03:35:44 pm
Excellent work zlymex. It would be interesting to see the noise floor with a few values of R1, up to say 1M, to measure the actual input noise current.
Thanks. Did you mean to change C1 as well togther with R1?
I can at least do 22uF-100k pair and 2.2uF-1M pair with no difficulty.
 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on May 16, 2016, 07:38:41 pm
The interest is likely getting input current noise data on the OPs. So it's about changing R1 to a large value (like 1 M or 10 M) and keep the input open, so C1 does not matter.

Input current noise data for OPs seem to be not that reliable, so measured data are of interest. It might be good to know anyway how much input current noise the circuit has, just to know the background. Though I don't think it will not be much of a concern with just 1 K at the input.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on May 17, 2016, 03:11:07 am
Okey, I managed to run the circuit at R1=10 Meg and C1 open. It is not as easy as I thought because the voltage created by Ib on R1 is very large and easily saturated the output.
Anyway, the reading is somewhat between 27uVpp and 32uVpp, see chart attached.
The noise of a 10M resistor is roughly 6uVpp @10Hz bandwidth, therefore that 30uVpp is largely contributed by the noise current of the opamp which will be less than 32uVpp/10M/1.4 = 2.3pApp per opamp, smaller than specified 10pApp.

I also measured the Ib = 150pA
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: alanambrose on May 31, 2016, 04:06:23 am
Hey zlymex,

Is this the low leakage capacitor you're using?

http://www.mouser.co.uk/ProductDetail/United-Chemi-Con/EKMG350ELL222ML25S/?qs=sGAEpiMZZMtZ1n0r9vR22WE9Am08kdJyz%252bhaf7%252bY9CY%3d (http://www.mouser.co.uk/ProductDetail/United-Chemi-Con/EKMG350ELL222ML25S/?qs=sGAEpiMZZMtZ1n0r9vR22WE9Am08kdJyz%252bhaf7%252bY9CY%3d)

Regards, Alan
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on May 31, 2016, 11:03:18 am
Hey zlymex,

Is this the low leakage capacitor you're using?

http://www.mouser.co.uk/ProductDetail/United-Chemi-Con/EKMG350ELL222ML25S/?qs=sGAEpiMZZMtZ1n0r9vR22WE9Am08kdJyz%252bhaf7%252bY9CY%3d (http://www.mouser.co.uk/ProductDetail/United-Chemi-Con/EKMG350ELL222ML25S/?qs=sGAEpiMZZMtZ1n0r9vR22WE9Am08kdJyz%252bhaf7%252bY9CY%3d)

Regards, Alan

Yes, the type is KM, also rated at 105 deg C, but not low leakage type.

I choose this KM type because that is the thing immediate available to me, and I selected the one from a batch of five(according to minimum leakage).
Recently, I tested more capacitors of 470uF from Nitsuko(25V EL(M) type), Nippon Chemi-Con(35V SME type), Nichicon(25V VX(M) type), they are all very good in leakage current, but again should be selected in about 1 to 3 ratio. After apply 10V for a day, leakage current of about 50% of those capacitors is below 2nA.

Edit: someone suggested to me to use low leakage type(0.002CV leakage current) such as ELNA RLB, Nichicon KL or UKL
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: alanambrose on June 01, 2016, 07:57:06 pm
Thanks zylmex for the capacitor info. Re Nichicon UKL...

>>> After 1 minute's (for case size 10 × 12.5 or smaller) or 2 minutes' (for case size 10 × 16 or larger) application of rated voltage at 20°C, leakage current is not more than 0.002CV or 0.2 (µA) whichever is greater.

For 2,200uF / 50V that's ~220µA - so the spec isn't that helpful. Will order a few in to test. The Chemicon equivalents btw seem to be their LLA series.

Regards, Alan

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on June 02, 2016, 01:30:26 am
.......
For 2,200uF / 50V that's ~220µA - so the spec isn't that helpful. .............
That's true. However, the actual leakage is much smaller than specified for all the capacitors I tested. The KM type I use is specified as 0.03CV, 15 times worse than the low leakage type(0.002CV), so we can expect better performance for low leakage type.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on June 02, 2016, 02:06:26 am
Leakage testing takes some time. So it is not practical to do the leakage test on every cap produced. Thus leakage specs are very conservative so that even the worst samples will meet the specs - for most applications the leakage is not critical. So there is not that much demand for caps with low leakage specs.
Title: Re: DIY low frenquency noise meter
Post by: splin on July 19, 2016, 01:59:00 am
I mean Linear did the same thing for their filters in AN124f where they choose 1300uF-1.2k and 165uF-10k for 0.1Hz HPF.

Ok good point: that explains (partly) why for the LTC6655 (5V) I get different results against the data sheet.
Datasheet value is 0.25ppmpp = 1.25uVpp.
whereas I got 2.2uVpp for the MSOP and 2.7uVpp for the LS8-package.

Of course LT will not change its cirquit so they would have to update their datasheets with higher 1/f noise levels.

I don't think this is right. I simulated the AN124f filter and it's not far off: as you can see in the picture (normalized to 0dB gain) the -3dB points are at .192Hz and 9Hz which shouldn't reduce the total noise by much more than about 8% compared to .1 to 10Hz.

I think the confusion that has arisen is that zlymex was pointing out above that AN124 .1Hz HPF is incorrect and I'm guessing you assumed that AN124 LPF also had the same problem as zlymex's original LPF with a cutoff at 6.5Hz. It doesn't really matter of course how it arose but it means that there must be some other explanation of the discrepancies between the LTC6655 specs and the measurements.

zlymex measured the LTC6655 noise, in reply 2, as approx .33ppm compared to the .25ppm spec, (33% above spec) but I assume that was using the .16Hz - 6.5Hz filter (as calculated by Alex) - the schematic has been updated since but presumably the results are still from the original circuit. I estimate that the bandwidth reduction should reduce the measured noise by about 15% which means the .1 - 10Hz measured noise would be 33%/.85 = 39% above the spec. The .25ppm spec figure is as usual a typical figure so the part still meets the specs.

Andreas's measurements above, at 76% to 116% above the typical are much worse, and whilst technically within spec are very disappointing for a part which is expressly marketed as being low noise. I estimate that the .14Hz -3dB HPF error compared to .1Hz would only have an error of around 2%, so pretty negligible. So the question is, was Andreas particularly unlucky with the various parts he tested, or did the LT engineer/marketing bod who wrote the datasheet previously work at Vishay  >:D or is Andreas's LTC6655 test setup / layout inducing excess noise for some reason?

To estimate the integrated 1/f noise with differing bandwidths, I used the formulae:

Vnoise rms = Vnw x sqrt(FC x ln (FH/FL))

where:

FC = 1/f cutoff frequency
Vnw = noise density well above FC
FH = HPF -3dB frequency
FL  = LPF -3dB frequency

Vnw is used for frequencies above FC when FH > FC

The only voltage reference datasheets I could easily find showing 1/f noise spectrums were the LTZ1000 and the ADR4520 (bandgap), both of which showed FC to be around 2Hz and the AD584 with FC nearer 150Hz. I used 2Hz in the above estimates; increasing it to 150Hz increases the impact of the LPF errors but decreases the HPF errors. Eg. The AN124 bandwidth errors only change the integrated noise from 92% to 91% (of .1 to 10Hz) when FC is increased from 2 to 150Hz.
Title: Re: DIY low frenquency noise meter
Post by: David Hess on July 19, 2016, 02:20:11 pm
BTW--- the LTC2057 suppresses 1/f noise.  It's LF noise spec is from DC-10Hz because of this.

Chopper stabilized amplifiers have flat 1/f noise to DC but higher broadband noise.  You can combine a chopper stabilized amplifier with normal amplifier to get the best noise characteristics of both.
Title: Re: DIY low frenquency noise meter
Post by: Andreas on July 19, 2016, 03:19:52 pm

 or is Andreas's LTC6655 test setup / layout inducing excess noise for some reason?


Hello,

thanks for the analyzing in detail.

The only thing in the test setup that I can imagine is probably the power supply voltage used.
The datasheet specs VREF+0.5V for all measurements.
Whereas I am typically using 9 or 10.2V for my measurements.
Unfortunately I did not record the power supply voltage.

And the LTC6655 is heating a lot with the 5 mA consumption.

I will do a comparison at different supply voltages (perhaps at the week end).

With best regards

Andreas

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on July 19, 2016, 08:19:39 pm
The low frequency noise can vary quite a lot from one sample to another. Its also rather slow an thus expensive to measure. Even with expensive parts like the LTZ1000, there are good ones an bad ones with way more LF noise than typical specs. So it really makes sense to have a system for LF noise measurements.

Besides electronic LF noise, there can also be thermal noise from turbulant air flow and thermal EMF and similar. This can look rather similar to 1/f noise.

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: splin on July 20, 2016, 03:10:41 am
The low frequency noise can vary quite a lot from one sample to another. Its also rather slow an thus expensive to measure. Even with expensive parts like the LTZ1000, there are good ones an bad ones with way more LF noise than typical specs. So it really makes sense to have a system for LF noise measurements.

True but the LTZ1000s are *very* expensive and given that noise is a very important characteristic, a close second to stability, then I don't think it is unreasonable that they should be 100% tested for noise. And since LT specify a maximum of 2uVpp, 1.2uV typical it looks like they have the same opinion. Of course you could still get parts exceeding the maximum but according to TI:

"All data sheet specs are usually obtained using a +/-3 sigma truncation of a typically Gaussian distribution of parts over process variations".

If your application demands parts that are more tightly specced than the datasheet maximum, or 3-sigma probability is not adequate of course you will need to test each part.

The LTC6655 are a lot cheaper, but still relatively expensive, so given the major headline feature is its very low noise, I don't think it would be unreasonable for some sort of noise screening to be performed - even if it were a very quick, and hence cheap, test for HF noise and LF noise at say 10Hz. LT on the other hand (like the vast majority of voltage references from all manufacturers) don't even bother to specify a maximum so in this case they don't agree. One would hope that in reality that the manufacturing process is well enough controlled, along with periodic QA testing, to ensure that the majority of parts do not exceed a reasonable multiple of the typical figure. The problem is what is reasonable? Given Andreas's noise measurements so far I personally would be a bit wary of the LTC6655 - but it is a very small sample.

Quote
Besides electronic LF noise, there can also be thermal noise from turbulant air flow and thermal EMF and similar. This can look rather similar to 1/f noise.

True, but Andreas's results for various other references have been roughly in line with the datasheet specifications which suggest that his procedures and test setup are good enough and have the above issues under control. It is just the 6655 results which are unexpected hence the question as to whether they have a particular problem in his setup such as inadequate decoupling, instability/HF oscillations etc.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: David Hess on July 20, 2016, 06:01:16 am
The low frequency noise can vary quite a lot from one sample to another. Its also rather slow an thus expensive to measure. Even with expensive parts like the LTZ1000, there are good ones an bad ones with way more LF noise than typical specs. So it really makes sense to have a system for LF noise measurements.

True but the LTZ1000s are *very* expensive and given that noise is a very important characteristic, a close second to stability, then I don't think it is unreasonable that they should be 100% tested for noise. And since LT specify a maximum of 2uVpp, 1.2uV typical it looks like they have the same opinion. Of course you could still get parts exceeding the maximum but according to TI:

"All data sheet specs are usually obtained using a +/-3 sigma truncation of a typically Gaussian distribution of parts over process variations".

If your application demands parts that are more tightly specced than the datasheet maximum, or 3-sigma probability is not adequate of course you will need to test each part.

I remember seeing a note from Linear Technology about contacting them for special noise (or drift?) grading of LTZ1000 references but a quick search did not find it.

Back when popcorn or burst noise was a problem, they could not test for it because it took too long;  I have seen this myself in parts where testing for it would have required hours to days.  Low noise testing for operational amplifiers is also a problem so at least for low cost parts, if they test for it at all they rely on its correlation to high frequency noise which is much faster to test.

The warmup time for the LTZ1000 would require low frequency noise testing to wait 100s of seconds but given the price, I wonder why this is not economical.  Aren't these burned in anyway?

Quote
True, but Andreas's results for various other references have been roughly in line with the datasheet specifications which suggest that his procedures and test setup are good enough and have the above issues under control. It is just the 6655 results which are unexpected hence the question as to whether they have a particular problem in his setup such as inadequate decoupling, instability/HF oscillations etc.

The LTC6655 datasheet discusses some pretty strict requirements for output capacitance with a 10uF film capacitor being about optimum but their noise test example uses 1uF which is below the 2.7uF minimum that they recommend.  It is not clear what if any effect this has on low frequency noise.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on July 20, 2016, 07:29:47 am

The LTC6655 datasheet discusses some pretty strict requirements for output capacitance with a 10uF film capacitor being about optimum but their noise test example uses 1uF which is below the 2.7uF minimum that they recommend.  It is not clear what if any effect this has on low frequency noise.


Hello,

yes I also read the special requirements of special capacitors.
Between the lines: manufactured by virgins with golden hair in a full moon night from Teflon and oxygen free copper.

Edit: Unfortunately I am using only poor mans combination of a 10uV Ta (size A) in parallel with a 100nF 1206 X7R capacitor.

But also the supply voltage has a large influence.
I did a comparison with stabilized voltage of 10.2V and 5.66V at the input of the reference on my sample with the LS8 package.

This gives a factor 1.32 difference when averaging over 10 measurements with 10 seconds each.
10.2V gives 2.717 uVpp
5.66V gives 2.044 uVpp
and also the AC rms voltage (nVeff) has around factor 1.32 difference.

With best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: David Hess on July 20, 2016, 08:46:10 am
The LTC6655 datasheet discusses some pretty strict requirements for output capacitance with a 10uF film capacitor being about optimum but their noise test example uses 1uF which is below the 2.7uF minimum that they recommend.  It is not clear what if any effect this has on low frequency noise.

yes I also read the special requirements of special capacitors.
Between the lines: manufactured by virgins with golden hair in a full moon night from Teflon and oxygen free copper.

Edit: Unfortunately I am using only poor mans combination of a 10uV Ta (size A) in parallel with a 100nF 1206 X7R capacitor.

The requirements did not strike me as quite that bad but film capacitors of that size are annoyingly large.  The low ESR requirement could be met with a polymer aluminum electrolytic or maybe a polymer tantalum capacitor.

I wonder what about the LTC6655 made for such high capacitance and low ESR requirements.  If a normal tantalum and ceramic combination was suitable, I think they would have said so.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on July 21, 2016, 02:44:49 am
I think the combination of two different caps might actually be a good idea. It's a little like with lab power supply circuits: a certain low ESR capacitance is needed to prevent high frequency (e.g. 100 kHz) instability. For the bulk capacitance the very low ESR should is not be really needed and some ESR (e.g. 0.1 Ohms range) might even help. At least this is what the output impedance curve from the data-sheet suggest. It's only a large capacitance with high ESR (e.g. 100 µF with  more than 1 Ohm ESR)  that is more of a trouble.

However I would avoid both tantalum and X7R: the tantalum caps might cause noise spikes, at least some of them do. The X7R can be slightly piezo electric and thus pic up mechanical noise. I would prefer low ESR Al (e.g. 100 µF), maybe polymer and a small film cap (e.g. MKS 220 nF).

Anyway the capacitors should not have that much influence at LF noise - it might make a difference in the kHz range.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on July 21, 2016, 01:38:35 pm
.......
Besides electronic LF noise, there can also be thermal noise from turbulant air flow and thermal EMF and similar. This can look rather similar to 1/f noise.

That's especially true for references with very low LF noise such as 2DW234.
When I first measured this Zener, the noise is higher. The noise only reached to around 0.3uVpp when I wind shielded it with a lot of tissues.
Title: Re: DIY low frenquency noise meter
Post by: zlymex on July 21, 2016, 01:47:12 pm
......
zlymex measured the LTC6655 noise, in reply 2, as approx .33ppm compared to the .25ppm spec, (33% above spec) but I assume that was using the .16Hz - 6.5Hz filter (as calculated by Alex) - the schematic has been updated since but presumably the results are still from the original circuit. I estimate that the bandwidth reduction should reduce the measured noise by about 15% which means the .1 - 10Hz measured noise would be 33%/.85 = 39% above the spec. The .25ppm spec figure is as usual a typical figure so the part still meets the specs.

My measurement of LTC6655 in reply 2 is new.
I used light-blue remarks before(for the .16Hz - 6.5Hz filter), then I changed to purple remarks for the correct bandwidth of  .1 - 10Hz.
Since the result of both LTC6655-1.25 and LTC6655-2.5 are purple color marked, it is the right bandwidth.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on July 21, 2016, 02:24:15 pm
.....
The LTC6655 datasheet discusses some pretty strict requirements for output capacitance with a 10uF film capacitor being about optimum but their noise test example uses 1uF which is below the 2.7uF minimum that they recommend.  It is not clear what if any effect this has on low frequency noise.

The output capacitor of a reference normally has two uses, one is to suppress HF noise, the other is to prevent oscillation. Either way, it has not much to do with LF noise especially below 1Hz.
For function of suppress HF noise, the capacitor can be omitted.
For function of preventing oscillation, the capacitor in the datasheet often has a very large allowance.
For function of preventing oscillation, it needs the ESR of the output capacitor to form a zero(as against pole), therefore, it is usually not "the lower the ESR the better".
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: David Hess on July 21, 2016, 03:19:08 pm
...

For function of preventing oscillation, the capacitor in the datasheet often has a very large allowance.
For function of preventing oscillation, it needs the ESR of the output capacitor to form a zero(as against pole), therefore, it is usually not "the lower the ESR the better".

But in this case it is "the lower the ESR the better" which I found to be very unusual.  What about the LTC6655 requires such a large and low ESR capacitor?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on July 21, 2016, 04:00:17 pm
...

For function of preventing oscillation, the capacitor in the datasheet often has a very large allowance.
For function of preventing oscillation, it needs the ESR of the output capacitor to form a zero(as against pole), therefore, it is usually not "the lower the ESR the better".

But in this case it is "the lower the ESR the better" which I found to be very unusual.  What about the LTC6655 requires such a large and low ESR capacitor?
It perhaps not "the lower the ESR the better" in the case of LTC6655. When Cout=100uF is used, there is a noise peak at about 3Hz which inferior than 10uF where the curve is flat below 30Hz. Presumably the ESR of 100uF capacitor at 3Hz is smaller than 10uF.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on July 21, 2016, 05:13:33 pm
The noise peak is at 3 kHz, not 3 Hz - so not really a problem with LF noise.
This noise peak comes from a resonance from the output capacitance and the output impedance of the reference chip. To reduce / dampen the resonance, a higher loss /  ESR (but still not that much) of the capacitor in the kHz range is desirable. However to prevent trouble at higher frequencies  a low ESR at higher frequency is needed. A singe simple capacitor can not provide this, but a combination of a possibly small low ESR cap (e.g. 1 µF foil, ceramic) and a large cap with moderate ESR (e.g. 100 µF with  0.1-0.5 Ohms ESR).
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on July 21, 2016, 05:57:07 pm
The noise peak is at 3 kHz, not 3 Hz - so not really a problem with LF noise.
....
Oh yes.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: David Hess on July 22, 2016, 04:08:22 am
The noise peaking is exactly what I would expect if the ESR is too low but that still does not answer the question.  What effect on the low frequency noise or other performance characteristic led LT to recommend a large capacitor with such a low ESR?

If a good solid tantalum or aluminum electrolytic capacitor in parallel with a smaller film or ceramic capacitor was suitable, then why didn't they recommend that less expensive solution?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Edwin G. Pettis on July 22, 2016, 08:25:53 am
Ceramic capacitors are noisy, voltage sensitive and have a host of ills, they should not be used in any noise sensitive circuit.  I suggest a review of LT's notes, particularly from Jim Williams on ceramic capacitors.

A short listing of ills:

They are one of the noisiest capacitors made.
They are voltage sensitive, most type's capacitance varies with applied voltage and frequency.
They are mechanically sensitive, generating spurious noise spikes.

While most capacitor types exhibit an inverse noise vs. capacitance curve, most ceramics do not follow this curve and are noisy all over the place.

There are 50V polypropylene capacitors available which are reasonably small, 63V might be slightly more common.  Yes they are more expensive than aluminum or tantalum but do not have spurious noise spikes if that is important to the circuit.

If you have ceramics in your reference circuit, remove them!
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: acbern on July 22, 2016, 06:38:39 pm
This only applies to ferroelectric material capacitors, the use of COG caps is ok for this purpose. Getting SMDs in 0.47uF is no problem. For the higher capacitances, Oscons are recommendable to their supperior AC behaviour.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on August 16, 2016, 07:51:34 pm
Update.
One of my friends(enjoydiy) made some compact noise meters based on this.
Magnification: 10,000
Band: 0.1Hz - 10Hz (-3dB)
Main Apamp: ADA4528-2ARMZ
HPF: 2nd order
LPF: 4th order Butterworth
Input capacitor: Nichicon UKL 1000uV/50V(low leakage type electrolytic)
Power: one 14500 rechargeble Lithium battery(AA size), 3.0-4.2V
Charge port: micro USB
Case: 25*40*83.5mm
Floor noise: 100nVp-p
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: lukier on August 16, 2016, 08:13:10 pm
Very nice compact design. Is your friend planning to sell some by any chance?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on August 16, 2016, 09:23:32 pm
Very nice compact design. Is your friend planning to sell some by any chance?
Those units in the first photo have already divided up among his friends. I didn't hear anything about his selling plan yet.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Alex Nikitin on August 16, 2016, 09:50:07 pm
I would buy one as well.

Cheers

Alex
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: pelule on August 17, 2016, 04:04:50 am
I would buy one as well.

Cheers
PeLuLe
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on August 17, 2016, 10:05:21 am
And I would buy 2.
Does your friend plan to release gerbers? I could make boards :-]
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on August 17, 2016, 02:27:47 pm

Input capacitor: Nichicon UKL 1000uV/50V(low leakage type electrolytic)

Floor noise: 100nVp-p
Hello,

nice design.
What input impedance do you have?

(seems to be above 3 k-Ohms)

With best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on August 17, 2016, 04:50:29 pm
Hmmmm...  A new op-amp from Linear Tech might be useful for this task:

LT6081 (http://cds.linear.com/docs/en/datasheet/6018f.pdf)

-Ken
Thanks for the info. This opamp seems to have the same or better voltage noise performance than LME49990 especially below 1Hz. But the  low frequency current noise is worse than that(Unbalanced).
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on August 17, 2016, 05:01:31 pm

Input capacitor: Nichicon UKL 1000uV/50V(low leakage type electrolytic)

Floor noise: 100nVp-p
Hello,

nice design.
What input impedance do you have?

(seems to be above 3 k-Ohms)

With best regards

Andreas

This is the input and amp part, plus bandwidth simulation(of the whole meter).
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: splin on August 18, 2016, 03:10:20 am
If you only care about the DC..10Hz noise, then you could have up to about 10K [total] input resistance without spoiling the 30nVpp noise specs.

I make it < 500 ohms @ 10Hz (2.5pA/rt(Hz) v 1.2nV/rt(Hz)),  < 175ohms @ 1Hz (8pA/rt(Hz) v 1.4nV/rt(Hz)) and < 100ohms @ .1Hz providing you balance the inputs.[/quote]
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: splin on August 18, 2016, 05:06:38 am
If you are looking at the wide band noise-- yes you are right.  If you are only concerned with DC-10Hz, then the current noise [@ 0.1 to 10Hz, in pApp] divides into the LF noise spec-- and that is 30nVpp for this amp.

Sorry if I didn't make it clear - I was referring to the current and voltage noise density graphs on page 7 from which came the

Quote
I make it < 500 ohms @ 10Hz (2.5pA/rt(Hz) v 1.2nV/rt(Hz)),  < 175ohms @ 1Hz (8pA/rt(Hz) v 1.4nV/rt(Hz)) and < 100ohms @ .1Hz providing you balance the inputs.
[/quote]

I didn't bother to quote the .1Hz figures but for completeness they are approx 30pA/rt(Hz) and 30nV/rt(Hz). I also haven't integrated the current noise over .1 to 10Hz but it will clearly significantly exceed the 5nV rms (30nVpp) opamp voltage noise with 500ohms source impedance.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on August 18, 2016, 05:34:32 am
The LT6081 is similar to the LT1028: good for a really low impedance source only, e.g. less than 500 Ohms (balanced) at higher frequencies an less than about 100 Ohms in the LF range. So this is not practical with an AC coupled low frequency input.

Having balanced inputs often also adds extra noise from the extra resistor to balace the inputs.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on August 18, 2016, 09:35:19 am
Anyone would be willing to make up schematics with abovementioned LT6081 to try? :)
I'd get PCBs and prototype assembly done for it to test in HW, including extra PCBs for contributors.
Plan to build something to test my LTZ's anyway, so one way or another I'd be making something, but rather make something reasonable from folks who understand all this opamp usage better than me.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on August 18, 2016, 11:01:39 am
LME49990(dated 2009) was the improvement of LT1028. LT6081 is the same or worse than LME49990 as far as the noise is concerned. Both of them having the same low frequency noise of 30uVp-p, but the current noise of LT6081(unbalanced) is worth than LME49990.

I've considered using LT1028 or LME49990 as the first stage of the noise meter, but failed to get anything better than current ADA4528-2 mainly because the very large Ib and the incapability to make balanced input for one stage amplification. The typical Ib(Abs) of LT6081 is very similar to LT1028 and LME49990.
Title: Re: DIY low frenquency noise meter
Post by: BU508A on August 18, 2016, 03:29:16 pm
Hi,

Chopper stabilized amplifiers have flat 1/f noise to DC but higher broadband noise.  You can combine a chopper stabilized amplifier with normal amplifier to get the best noise characteristics of both.

There was a design note (DN 36) from LT which describes such a combination.

Also there was a very interesting design note (DN 42) from LT with a comparison of chopper amplifiers vs. bipolar OpAmps.

And at last there is an application note (AN 20) with some considerations about instrumentation low pass filters with some comments in it about capacitors.

I've attached all PDF documents here below except the AN20, because it was too big (2.7MByte). Here is the link: http://www.linear.com/docs/4115 (http://www.linear.com/docs/4115)

Andreas


Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on September 27, 2016, 10:41:04 pm
Alright, I cannot let zlymex get all the glory, so let get some more nanovolts flying.

How about we build few known LNA designs to test in practical environments?

Two with quarupled opamps. LNA1 with Analog instrumentation amp, AD8428. Paper about it here (http://www.analog.com/library/analogdialogue/archives/49-05/nanovolt.html). Second is discrete one, using ADA4522-4, copycat of zlymex's design.

(https://xdevs.com/doc/xDevs.com/KXULNA/img/lna_mk1_1.png) (https://xdevs.com/doc/xDevs.com/KXULNA/img/lna_mk1.png) (https://xdevs.com/doc/xDevs.com/KXULNA/img/lna_mk2_1.png) (https://xdevs.com/doc/xDevs.com/KXULNA/img/lna_mk2.png)

And third one is classic from Jim with few changes, based off Linear AN124.

(https://xdevs.com/doc/xDevs.com/KXULNA/img/lna_mk3_1.png) (https://xdevs.com/doc/xDevs.com/KXULNA/img/lna_mk3.png)

I do realize gain of these amps are different, so perhaps option to set equal gain should be considered, or just leave them as is, using "black box" test approach, to see which one has less noise.

 :-DMM
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on September 28, 2016, 12:09:58 am
The AD8428 is a BJT based design and thus has high current noise. So like the LT1028 it is good for low impedance sources (e.g. < 1 K, better even < 300 Ohms) only and thus not a good choice for something like an AC coupled low frequency amplifier. Having more units in parallel only makes it even lower impedance. The AD8428 might be an interesting option at high frequencies.

For combining the output of 4 amplifiers there is no need for precision resistors - just about any resistor will do - if you want even carbon type and 20% tolerance.

For an AC coupled LF amplifier it is usually better to have the coss over frequency of the input RC lower (and thus higher resistance) and set the lower frequency limit at a later stage or in software.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on September 28, 2016, 01:43:10 am
Those are very nice design of TiN, though I worried about the current noise for MK I circuit as well.
AD8428 is specified as 150pAp-p LF current noise, parallel four makes 300pAp-p, this will create 150nVp-p voltage noise on an 500 Ohm input impedance, and this noise voltage will overwhelm the 40nVp-p typical voltage noise of the opamp(20nVp-p if 4 in parallel).
However, the current noise spec of opamps may well be wrong(may be either under-stated or over-stated), so it is worth trying it out.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on September 28, 2016, 02:23:48 am
Let's say nice design once it's on the table and working nice. I'd be careful on cheering without any practical validation, yet.

In such case just with few parts swap it could be then DC coupled and used as null-detector between referenced for example. :)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zlymex on September 28, 2016, 02:51:31 am
That's right, DC coupled LNA. I've been considering DIY an altra-low noise(<160nVp-p), variable voltage reference(0.9V-10.1V) as the base for that.
This will allow opamps with small voltage noise but large current noise to be used as the LNA.
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=258889;image)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on September 28, 2016, 03:14:54 am
With a BJT type amplifier the high current noise is normal and well understood. So I won't have much hope for a miracle. There might be quite some scattering in the 1/f part, but this is more like a few bad parts, not a few magic ones that don't follow established theory.

Current noise in AZ OPs is a much more complicated thing as noise could depend on details like capacitance at the inputs or even decoupling and supply voltage. Also measurements are more tricky at lower levels and not all chips might be the same here, as the bias current can show quite some scattering. So with AZ OP the current noise specs can be tricky and may have errors. So here you can be lucky and find some that that are much better than specs - but also overly optimistic specs.

So the AD8428 is more like a good amplifier for the 100 Hz - 100 kHz Band, or maybe with DC coupling for the 1 Hz - range. However in the low range, there will be trouble from 1/f noise and chopper amps should be better. So not really good for a null meter either.

For the AN124 like design the 2N4393 JFET is an unusual choice. A good low noise JFET would be the BF862 (at least some of them have good 1/f noise). The ADA4822 does not make much sense with a 100 K input resistor - it needs smaller resistors (and thus larger caps) to take full advantage of the low noise specs of the OP. The choice of resistors on the drain side of the JFETs is also a little strange (I would expect more equal values for R33 and R34). Also near 10 mA looks like a lot of current for the FETs. At so much current I would consider limiting the voltage by using a cascode or bootstrapping to limit the heating and thus thermal "noise". 1/f noise often gets better with lower current per transistor and more of them in parallel. With the JFETs having a few in parallel is not that bad (except for costs and matching), as current noise is usually still low.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on October 26, 2016, 05:25:15 pm
Hello,

I don´t know what exactly you want to do. But be carefully.

When lowering the input impedance of your amplifier too much you will get a voltage divider
by the output resistance of your noise source (typical up to 20 Ohms for a LTZ1000) and your input resistance.
So around 1000 Ohms are a practical lower limit.

With best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on October 31, 2016, 04:26:36 am
Well, finally I got around and did a layout of the board for MK-I (40x100mm FR4 4L).

(https://xdevs.com/doc/xDevs.com/KXULNA/ulna_a_1.png) (https://xdevs.com/doc/xDevs.com/KXULNA/ulna_a.png)

Green shape is solder-mask opening, so metal shield cage can be soldered on top to provide shielding and protection from air drafts.
Signals can go via SMT EMI pass-thru filters.

Schematics in PDF (http://xdevs.com/doc/xDevs.com/KXULNA/ULNA-A.pdf)

I'll revise MK2 and MK3, and will do similar size boards for them too.

Actually whole process was recorded and livestreamed on YT, so you can watch the whole thing in realtime, as well as my mambling.

Part 1, 2hour 28 minutes
Part 1, 2hour 28 minutes (https://youtu.be/cAjXOYWSiTM)

Part 2, 4 hours (max limit on YT for HD, so first 1hour is trunicated :((
Part 2, 5  hours (http://www.youtube.com/watch?v=viQFq7XGCxE)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: quarks on October 31, 2016, 04:51:41 am
Somehow I totally missed this, so now it is bookmarked.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Nuno_pt on October 31, 2016, 04:54:22 am
The mumbling from Illya is the best. :-)

Looks like I've 3 more boards to build slowly.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on October 31, 2016, 05:17:03 am
On how much of the OPs one can / should use in parallel depends on the ration of voltage noise to current noise. This ratio gives the source impedance that has lowest equivalent noise power or the best noise figure.
Using more OPs in parallel can shift the optimum to lower source impedance.
So more OP in parallel only make sense if the source (here the RC circuit) is low impedance.

Here we are looking at low frequency noise so one could take the 0.1 Hz to 10 Hz noise. Using the 1 kHz noise density can give slightly different numbers.

So the ADA4522 specs are 117 nV_pp and 16 pA_pp for the 0.1 to 10 Hz range. Thus an optimum source resistance of 7.3 K Ohms. However noise current specs on the AZ OPs are a little tricky - so the real world samples might be a little different.
With the AC coupled input the impedance is frequency dependent. As the noise is mainly white (not much, if any  1/f) it is more the higher frequencies (e.g. 1 or 10 Hz) that set the limit. It is the capacitor that sets the limit. For a 1 Hz frequency of interest each OP would like to have at least 20 µF of capacitance.
So it depends on the capacitor and the frequency of interest how many OPs in parallel make sense.

Similar things apply to BJT based OPs like the LT1007.
Here the LT1007 has best noise figure somewhere around 100 Ohms in the LF range (e.g. 10 nV and 100 pA at 0.1 Hz). In the higher frequency range the best noise figure is at about 600 Ohms. The AD8428 would be similar or lower in impedance - so it is very limited use in using them in parallel in the LF range.

As these amps have 1/f noise the lower frequencies are more relevant. So the LT1007 would like to have a quite large input cap, so more like 5000µF or more.

Paralleling might be more useful for JFET based OPs, like the OPA140. Even with something like 6-8 in parallel to get a similar 0.1 -10 Hz noise level as the ADA4522 the current noise should still be low in the LF range.

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on October 31, 2016, 05:25:45 am
I think it's cheap to try build all three variants and test them on practice using various DUTs at input. Also different input RC can be tested
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: VintageNut on November 01, 2016, 11:16:17 pm
I have a question about using the noise meter with an oscilloscope.

What is the noise floor of the Picoscope that is shown in the measurement in this thread?

The scope that I am using seems to have a noise floor of 640 uV p-p when set in 20MHz bandwidth mode. In 250 MHz bandwidth mode the scope noise is 1.4 mV p-p. The scope is set for 1X probe and is on the 1mV/div range which is the most sensitive range.

If the noise floor of the noise meter is somewhere between 90nV p-p and 170 nV p-p from reading this thread and some app notes on the web, the gain needed to be above the scope noise floor will be 10,000.

Does that sound correct?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on November 01, 2016, 11:38:09 pm
Hello,

the noise of the PicoScope 5000 series is specced to be 70uV RMS (450uVpp) in 16 Bit mode in 50mV range (5 mV/Div).
see datasheet:
https://www.picotech.com/download/datasheets/MM040.en-8.pdf (https://www.picotech.com/download/datasheets/MM040.en-8.pdf)

But you can reduce the bandwidth to further reduce the noise.
There is a 20 MHz hardware bandwidth filter and several software FIR filters.
Either resolution enhancement to 20 Bits (a 256 sliding average filter) or a configurable edge frequency (e.g. 1kHz).
With this filter the noise is at 15 uVpp referred to the input of the scope. See also:
http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg938724/#msg938724 (http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg938724/#msg938724)

The 10000:1 amplifier is needed to scale from nV to mV.
In my case the 10000:1 amplifier determines the noise floor around (100nVpp) with either 20 Bit resolution or the 1 kHz filter on the scope.

with best regards

Andreas

edit: 15uVpp scope noise alone with filter
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: VintageNut on November 02, 2016, 12:32:59 am
Hello,

the noise of the PicoScope 5000 series is specced to be 70uV RMS (450uVpp) in 16 Bit mode in 50mV range (5 mV/Div).
see datasheet:
https://www.picotech.com/download/datasheets/MM040.en-8.pdf (https://www.picotech.com/download/datasheets/MM040.en-8.pdf)

But you can reduce the bandwidth to further reduce the noise.
There is a 20 MHz hardware bandwidth filter and several software FIR filters.
Either resolution enhancement to 20 Bits (a 256 sliding average filter) or a configurable edge frequency (e.g. 1kHz).
With this filter the noise is at 15 uVpp referred to the input of the scope. See also:
http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg938724/#msg938724 (http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg938724/#msg938724)

The 10000:1 amplifier is needed to scale from nV to mV.
In my case the 10000:1 amplifier determines the noise floor around (100nVpp) with either 20 Bit resolution or the 1 kHz filter on the scope.

with best regards

Andreas

edit: 15uVpp scope noise alone with filter

Thank you. That is what I was looking to know. I can probably approximate this with the scope that I have.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: VintageNut on November 02, 2016, 10:03:39 am
I decided to see what the DMM7510 can do with the digitizer. Using 1000 samples/second, and a crappy banana plug with some cheap speaker wire as a short, the noise is 7.2 uV p-p. Respectable.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: VintageNut on November 03, 2016, 01:49:43 am
The DMM7510 cooked overnight and some today. Attached is the screen capture with statistics for the shorted input on the 100mV range.

10.04uV p-p, 4.02uV average, Std Dev 1.03uV, 33,000,000+ readings at 1000 samples per second, 9+ hours.

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on November 03, 2016, 02:50:56 am
To measure noise, the peak to peak reading is a little difficult, as it is by definition sensitive to singular events. So it will always be not that well reproducible. Today the better way to measure noise is using FFT and than measure the noise spectrum. A fast DMM like the 7510 is a good option here.
For a given frequency band (e.g. 0.1 -10 Hz as the typical for low frequency noise) one can also use the RMS value (= Std deviation) - this number is much better reproducible than the peak to peak value. To get the right bandwidth this would be more like 10 s windows with something like a 20 Hz sampling rate.

Doing measurements on very long times, there will be a mixture of noise and drift. To get rid of the drift one might want to add a lower frequency limit, by using a software high pass filter before calculation RMS values.
So the lone run gives a number for about the 0.03 mHz - 500 Hz range. The upper band limit depends on filters used in the DMM.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: VintageNut on November 03, 2016, 03:10:17 am
The raw buffer data is always available for the DMM7510. A post-processing filter can perform whatever the user wants.
I am not too worried about bandwidth. The noise meter that is being proposed has the band limit built-in.

On my oscilloscope the ratio of scope channel p-p noise to RMS is something like 10X, 600 uVp-p and 50 uV RMS. 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on November 03, 2016, 03:42:14 am
The ratio of peak to peak value to RMS value depends on the bandwidth (especially the ration of lower and upper limit), the length of time window and the type of noise (white or 1/f). The usual number is something like a factor of 6 or a little more (e.g. 7).

A ration of 10 for the scope should be due to the high bandwidth ratio (e.g 0.1 Hz - MHz).

A noise measurements needs to keep in mind the frequency band - otherwise the values are not comparable with other instruments. For a very accurate comparison one would also need to note the type of filter (e.g. 2nd or 3rd order, steepness) - there is more than just the band limit to characterize a filter. The concept of noise equivalent bandwidth only works it the type of noise is known (e.g 1/f, white or other).

The noise meter as shown has some band-filters build in, but there will be also limits from the DMM / scope. As shown there is only a first of second order high pass filter - this may not be enough if there is heavy 1/f (an maybe 1/f² part) noise. The initial AC coupling adds some noise in the transition region. So ideally there should be an extra filter (e.g digital in the DMM or PC)  to set the lower frequency limit instead of the initial RC combination.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: dr.diesel on November 03, 2016, 05:27:54 am
@VintageNut

I don't recall seeing your name in the DMM Noise Comparison thread here.  If you've not seen it make sure and check it out.   :-+
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: VintageNut on November 03, 2016, 01:31:52 pm
@VintageNut

I don't recall seeing your name in the DMM Noise Comparison thread here.  If you've not seen it make sure and check it out.   :-+

Ok, I am reading it now. Thanks
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: David Hess on November 04, 2016, 02:59:11 am
To measure noise, the peak to peak reading is a little difficult, as it is by definition sensitive to singular events. So it will always be not that well reproducible. Today the better way to measure noise is using FFT and than measure the noise spectrum. A fast DMM like the 7510 is a good option here.
For a given frequency band (e.g. 0.1 -10 Hz as the typical for low frequency noise) one can also use the RMS value (= Std deviation) - this number is much better reproducible than the peak to peak value. To get the right bandwidth this would be more like 10 s windows with something like a 20 Hz sampling rate.

I have always gotten great results using the standard deviation technique to make low frequency RMS noise measurements.  While the sin(x)/x response of most DC voltmeters limits the frequency response (see below), aliasing does not; take away half of the samples and the uncertainty goes up but the standard deviation does not change.  Sampling RF voltmeters take advantage of this to make GHz+ wideband RMS measurements while undersampling by millions of times.

Good peak to peak measurements can be made after amplification using a resetable analog peak to peak detector as described in one of Jim William's application notes or with a fast sampling voltmeter or DSO.

Quote
Doing measurements on very long times, there will be a mixture of noise and drift. To get rid of the drift one might want to add a lower frequency limit, by using a software high pass filter before calculation RMS values.
So the lone run gives a number for about the 0.03 mHz - 500 Hz range. The upper band limit depends on filters used in the DMM.

Besides any filtering, a DMM which integrates the input over a length of time like with a VFC, integrating, or delta-sigma converter, will have a sin(x)/x frequency response which is what is responsible for the 50 and 60 Hz normal mode rejection.  Also do not assume that the sample rate reveals the integration time because some ADCs like the recently discussed LTC2508-32 return multiple correlated samples over the integration time.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on November 17, 2016, 03:30:18 am
And here they come...

(https://xdevs.com/doc/xDevs.com/KXULNA/img/ulna_a_top_1.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/ulna_a_top.jpg)

Bit longer than my KX references, but same width/format.

(https://xdevs.com/doc/xDevs.com/KXULNA/img/ulna_a_bot_1.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/ulna_a_bot.jpg)

Bottom side.

(https://xdevs.com/doc/xDevs.com/KXULNA/img/ulna_a_stackup_1.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/ulna_a_stackup.jpg)

Stackup cutout ;)

Now need to get some caps, opamps and get the thing going!  :popcorn: :bullshit:
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: gamalot on November 18, 2016, 06:13:03 am
It's a little bit weird there are no pads for emitter pins of those transistors, rotate them 90deg CCW and have all 3 pads for each will be better.  :)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on November 24, 2016, 02:39:39 am
Got bit of time to assemble test module, using just one AD8248ARZ.

I think it's not working :)

(https://doc.xdevs.com/doc/xDevs.com/KXULNA/img/test_a/ulna_a1_1.jpg) (https://doc.xdevs.com/doc/xDevs.com/KXULNA/img/test_a/ulna_a1.jpg)

(https://doc.xdevs.com/doc/xDevs.com/KXULNA/img/test_a/ulna_t1_1.jpg) (https://doc.xdevs.com/doc/xDevs.com/KXULNA/img/test_a/ulna_t1.jpg)

(https://doc.xdevs.com/doc/xDevs.com/KXULNA/img/test_a/ulna_a2_1.jpg) (https://doc.xdevs.com/doc/xDevs.com/KXULNA/img/test_a/ulna_a2.jpg)

Reported values are way too low to be real. Tried 10VDC from HP 3245A and 10VDC from EDC MV106. It settles to final figure in matter of seconds. I thought cap need more time to charge?
I'm using Nichicon HE 1000uF 50V for now.

Tried +7/-7V from ultralow noise LDOs (my X1801 preamp power supply card, isolated) and today with paid of 9V batteries with same result.
Scope captures with 1Meg TCA-1MEG adapter and direct BNC cable. Attenuation on scope set to 2000:1.

(https://doc.xdevs.com/doc/xDevs.com/KXULNA/img/test_a/edc000.png)

(https://doc.xdevs.com/doc/xDevs.com/KXULNA/img/test_a/9823-new-ops005.png)

Seems there is no way around, but to build divider for generator, to make 0.1-1mV peak-peak waveforms to test with known signal.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: bktemp on November 24, 2016, 02:54:47 am
Could connecting the case of the capacitor to ground affect the operation?
The case is often not completely isolated from the capacitor, but has a potential slightly above the negative terminal. For example if you connect a capacitor to 12V, you will measure a couple of 100mV up to a few volts between case and negative terminal.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on December 06, 2016, 03:19:39 pm
Ok, amplifier works with DC coupling and 1Mohm load, so I think I'll need to add LT1028 on the output, so it can drive 50 ohm inputs of my scope directly.

Also I got magical wet slug 1300uF 30V cap (the correct type, XTV), initial check with 100Kohm in series on K2400 shown leakage ~5.2nA at 10VDC after night of soaking. Regular 75 ohm coax used. Looks promising so far.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on December 07, 2016, 12:05:57 am
The LT1028 is not really needed to drive the output. With it's high current noise it is even not that suitable to work with source of more than about 1 K (e.g. the resistors from combining the amps). There should be no need for an OP with an extra powerful output (e.g. more than 20 mA) - this might even be a danger to the 50 Ohms input.

Before adding a second amplifier, I would test for current noise as there might no be much use in this if current noise is too high.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on December 07, 2016, 01:09:11 am
Without buffer, AC coupled output cannot drive the 50 ohm load.

Here's actual schematics in sim:

(https://xdevs.com/doc/xDevs.com/KXULNA/test1/ulna_cir.png)

LTSpiceIV circuit file (https://xdevs.com/doc/xDevs.com/KXULNA/test1/ulna-a.asc).

AD8428 model using spice from AD site.

Symbol for LTSpice IV (https://xdevs.com/doc/xDevs.com/KXULNA/test1/AD8428.asy)
SPICE file (https://xdevs.com/doc/xDevs.com/KXULNA/test1/AD8428.cir)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on December 07, 2016, 01:55:58 am
I don't think a 11 µF cap to GND is a good diode idea at the output.

For AC coupling at the output with 11 µF and 100 K the LT1028 is not a good choice. It has way to much current noise. An OP07 or similar would be a better choice here.

The OPs output should also be insulated from a possible capacitive load. So add a series termination resistor. If DC / LF Gain needs to be 1 and independent of the 50 Ohm, add DC feedback from behind - just like an amplifier made to drive a capacitive load.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on December 07, 2016, 04:17:52 am
Diode? You meant C5/C6?

I have few LT1464A's, fits the case better?

(https://xdevs.com/doc/xDevs.com/KXULNA/ulna_a2.png)

Not everyday you have chance to see 707$ USD/1pcs (http://www.mouser.com/ProductDetail/Vishay-Tansitor/XTV138T030R4L/?qs=sGAEpiMZZMtZ1n0r9vR22eCGDrT0iCtFBpG6YJEZwp0%3d) capacitor, so here are the photos. Regular radial can caps and SMA connector are nearby for size comparison purpose.

(https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv1_1.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv1.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv2_1.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv2.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv3_1.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv3.jpg)

After 10 more hours leakage current stabilized around 3.2 nA at 10VDC from K2400.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on December 07, 2016, 05:51:14 am
The LT1494 is OK.

For lowest noise I would consider a larger resistor for R1 and maybe a smaller for R3, so that the lower frequency limit is set by R3*C3,C4 and not by R1*C1. This reduces the noise due to R1 for the lowest octave a little.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: VintageNut on December 07, 2016, 06:01:22 am
Hello Tin. Be aware that the 2400 operating at 3nA is not in its "sweet spot". I had occasion recently to use a 2400 to force 1nA and measure capacitor behavior. The 2400 did not respond well at 1nA. A model 236/237/238 responds much quicker on the same device forcing 1nA and letting the capacitor settle to a stable reading.

The length of time to stabilize the capacitor leakage that you observed may be the instrument and not the device.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on December 07, 2016, 06:49:37 am
Surely it's not sweet spot of 2400, but I have no time yet to power up 4142B and write program for it to run. I have more of these caps coming in few weeks, so will test and compare them using 41421A (1nA lowest range, 20fA resolution) once I get 'em.

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: doktor pyta on December 08, 2016, 09:40:55 am
@Tin
What is the reason of clamping input voltage to vcc+0.6V and vee-0.6V instead of simple clamping to +/-0.6V (no problem with leakage current of Q1 and Q2)?
AFAIK the gain of the amplifier will be high and 0.6V will saturate the amplifier.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on December 20, 2016, 09:34:25 pm
doktor pyta
Right, I somehow missed that fact. Good point, will update it to correct in next rev.

I also got greedy and got more of magical wet slug caps. Two of which are reverse polarity, meaning chassis is positive, not negative.

(https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv2_s_1.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv2_s.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv2_t_1.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv2_t.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv_box_1.jpg) (https://xdevs.com/doc/xDevs.com/KXULNA/img/xtv_box.jpg)

Also built a test box from cast enclosure with triax ports, so I can make semi-permanent setup to measure components using Agilent 4142B.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zhtoor on February 17, 2017, 05:58:29 pm
Great Work.

could you also please test noise on "popcorn" transistors like
PN2222, BC557, 2N3904 etc... in zener connection mode and also
some "standard" 5.1V to 6.8V Zeners?

regards.

Title: Re: DIY low frenquency noise meter
Post by: zhtoor on February 18, 2017, 02:43:39 pm
Some measurement result
Note also that the results here is only the noises that I tested using my DIY meter on particular voltage references that I possess.

1. Panasonic 3200uF/35V capacitor(I was told this is used for airbags), charge to 4.1V, 124nVpp
2. Panasonic NCR18650B lithium battery, charged to full more than six months ago, 4.1V, 115nVpp
3. A Chinese temperature compensated 6.3V zener, 2DW233, powered by 12V battery thru 1k resistor(5.7mA), 336nVpp, much better than a LTZ1000.
This ultra low noise characteristic of the 2DW23x series(from a particular maker) has been confirmed by many Chinese voltnuts before, but I don't believe this until I had my own test.
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=224382;image)
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=224466;image)
When current increased to 11.8mA, noise is reduce even further to 236nVpp.

4. Other measurement result is summarize in table below
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=225180;image)


5. Ordered by noise
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=225182;image)

6. Some words about 2DW23x
The one I tested is Diamond brand made by Shanghai 17th Radio Factory. I have a lot of other 2DW23x which are much inferior with noise figures ranging from 20uVpp to 100uVpp. The design and construction of this 2DW23x were completely changed although they still share the same datasheet.

Understandably the noise of a zener is inverse proportional to the square root of the zener current in theory,  and in practice I tested that 2DW233 follows this very well. The mystery is, how they achieve this kind of low noise?
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=225203;image)

I took apart one and took a photo with my card camera plus a magnifier. It seems to me that they are hand made because the die is not centered and wires are irregular.

great work.

could you please check out the following for noise performance?

MAT01AH (dual matched NPN from AD), one of the transistors connected as a zener in series with the other one as a diode,
and measuring noise from 1 to 10ma perhaps?

i suspect that this may rival all others in terms of noise and drift performance (with appropritately tuned current for zero TC).

regards.

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on February 18, 2017, 05:08:57 pm
Hello,

already did similar with a BCV62 (current mirror) with zener connected transistor and diode.
http://www.eevblog.com/forum/projects/low-cost-voltage-reference-experiment/msg633042/#msg633042 (http://www.eevblog.com/forum/projects/low-cost-voltage-reference-experiment/msg633042/#msg633042)

As they only guarantee the zener voltage is > 5V it will be usually not in a useful range for tempco compensation.
I got 8-9V on a simple test which is much too high against the ~6.25V needed.
So I think you will have to buy many MAT01 to find a pair with such low breakdown voltage.

You have many ideas.
Why not build your own low noise amplifier and report your measurement results?

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on February 18, 2017, 07:26:34 pm
Many "zener" based noise generators use the base - collector junction to as an extra noise zener. Zener-diodes with more than about 7 V are known to be quite noisy.

Measuring noise of zener diodes could be an interesting field for a noise testing equipment. The specs often don't give very much information on the noise. The interesting part is more the low frequency (e.g. < 10 Hz) part.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on February 18, 2017, 07:39:40 pm
they also use a very low current through the "zener" ...
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: zhtoor on February 19, 2017, 12:22:53 am
Hello,

already did similar with a BCV62 (current mirror) with zener connected transistor and diode.
http://www.eevblog.com/forum/projects/low-cost-voltage-reference-experiment/msg633042/#msg633042 (http://www.eevblog.com/forum/projects/low-cost-voltage-reference-experiment/msg633042/#msg633042)

As they only guarantee the zener voltage is > 5V it will be usually not in a useful range for tempco compensation.
I got 8-9V on a simple test which is much too high against the ~6.25V needed.
So I think you will have to buy many MAT01 to find a pair with such low breakdown voltage.

You have many ideas.
Why not build your own low noise amplifier and report your measurement results?

with best regards

Andreas



thanks for the input. a couple of points though.

1. since I am located in Pakistan, it is generally difficult to get hold of quality parts, so my proposals may sound funny.
2. yes noise testing is the key, that is probably the first piece of equipment any serious volt-nutter should have or build.
(point me in the right direction here.)
3. MAT01AH is already guaranteed to breakdown at the usual five to six volt region (zener as opposed to avalance breakdown).
4. i tend to think that if you did your selection from low-noise transistors otherwise, you may get much better long term drift specs,
   and MAT01AH is already pretty lownoise, along with guaranteed long term drift rates. (availability might be an issue).
   so using the MAT01AH's transistors as zener + diode combination and doing a sweep on the operating current while monitoring
   noise may be worthwhile.

regards, and thanks again for your input.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on March 01, 2017, 06:02:51 am
Incoming....

(https://xdevs.com/doc/_Metrology/Lymex_LNA/lln_parts_1.jpg) (https://xdevs.com/doc/_Metrology/Lymex_LNA/lln_parts.jpg)

(https://xdevs.com/doc/_Metrology/Lymex_LNA/lln_top_1.jpg) (https://xdevs.com/doc/_Metrology/Lymex_LNA/lln_top.jpg) (https://xdevs.com/doc/_Metrology/Lymex_LNA/lln_box_1.jpg) (https://xdevs.com/doc/_Metrology/Lymex_LNA/lln_box.jpg)

I need to patch my scope a bit, and prep the specimens for tests. It will be also interesting to try different capacitors I've bought together with this little neat preamp.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on March 01, 2017, 06:05:34 am
RSVD for tests.

Keithley 2400 Vsource , +9.5Vset.

(https://xdevs.com/doc/lymex_pipelie/ulna-1/k2400_9v5_1s.png)

(https://xdevs.com/doc/lymex_pipelie/ulna-1/k2400_9v5.png)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: lukier on March 01, 2017, 06:33:18 am
Looks like a nice & useful piece of kit. I wonder how it compares to trusty Tek's AM502 (with Jim Williams' mod for 10 Hz - see AN124).  :-+
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on March 01, 2017, 07:10:19 am
The amplifier(s) shown in this thread are considerably lower bandwidth (more like kHz) than the Tek AM502. They are also lower voltage noise in the LF range. However current noise can be higher and the input impedance is lower.

There is another tread on an JFET (BF862) based LNA that is more similar to the AM502 - though lower noise.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 01, 2017, 10:18:50 pm
Quote from: Andreas
Andreas

Just wondering if Andreas design that I repost here is still valid ... seems quite simple design if used with a "sensible" scope.



Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 02, 2017, 02:12:42 am
The amplifier(s) shown in this thread are considerably lower bandwidth (more like kHz) than the Tek AM502. They are also lower voltage noise in the LF range. However current noise can be higher and the input impedance is lower.

Can the AM502 be powered without the frame?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: lukier on March 02, 2017, 02:52:12 am
Can the AM502 be powered without the frame?

I guess so - haven't checked on AM502 because I have the frame now (TM501) but I did that with AM503 some time ago and you'll need AFAIR +-33.5V and +11.5V and two power transistors (one NPN, one PNP). Check TM501 & AM502 service manuals.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: David Hess on March 02, 2017, 03:24:12 am
Can the AM502 be powered without the frame?

The AM502 accepts +/-35 volts and +11 volts and regulates them down to +/-15 volts using a pair of external power transistors and +5 volts so it would not be too difficult to operate it with an external power supply.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 02, 2017, 06:39:05 am
Just wondering if Andreas design that I repost here is still valid ... seems quite simple design if used with a "sensible" scope.

Even my old HAMEG 412-5 does the job.
2 mV/div (maximum gain in 5mv/div range) gives 0.2uV/div referred to input.
So a LTZ1000A will give nearly full screen picture.

see also here:
http://www.eevblog.com/forum/projects/low-frequency-very-low-level-dc-biased-noise-measurements/msg658105/#msg658105 (http://www.eevblog.com/forum/projects/low-frequency-very-low-level-dc-biased-noise-measurements/msg658105/#msg658105)

With best regards

Andreas

Edit: link added
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 04, 2017, 10:07:10 am
Even my old HAMEG 412-5 does the job.

I was thinking 2mV was something not so common for a scope.

I got op amps. So now I'm exercising myself at drawing the PCB.

I started some capacitor testing too and I found an old Nichicon 16V 2200uF that went down to 8nA in a hour (10nA in 30 minutes) all the others seems to be very leakiiiiing.

Kinda like the idea to not have to wait forever for making the measurement. Will it be enough to use just this 2200uF?
Do I need to re-tune the filter?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 04, 2017, 04:29:21 pm
Hello,

be carefully when soldering. (leakage may go up again).
Standard 85 deg C Capacitors show usually a lower leakage than the 105 deg C types.
from 10 quality capacitors you should usually find 2 which are usable (after 2 days at 10 V).

What is not in the simulation schematic:
I have 2 * 100nF and 2*1000uF (the more leaking) for decoupling.
Between pre-amp and 2nd stage there is also a 100 Ohms in +/- power supply.
At the input there is a additionally 3K6 resistor (with shorting switch for measurement)
to charge up the input capacitor when connecting to a LTZ1000.
Otherwise you will damage the LTZ because the heater setpoint goes too long to infinite.

If you use only 2200uF you could use 1K5 as input pull down resistor.
But in this case you will have more noise floor due to current noise of the LT1037.
With only 2200uF you loose somewhat at the 0.1 Hz corner.
(not very much since the design is robust against input capacitor tolerance).

But if you use the cirquit only for comparative measurements this should not hurt much.

with best regards

Andreas

ps: and do not forget the cookies box on your BOM

http://www.lambertz-shop.de/gebaeck/gebackmischungen/composition-1000g.html (http://www.lambertz-shop.de/gebaeck/gebackmischungen/composition-1000g.html)

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 04, 2017, 07:58:27 pm
Yes and cookies can be used during assembling :-) too
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on March 04, 2017, 08:03:33 pm
A smaller coupling capacitor slightly increases the effect of current noise at the low frequency end. Increasing the capacitor to ground to keep the input frequency response about the same does not cause more noise:

The main effect is to increases the low frequency gain and thus more of the amplifier noise and more of the source noise is visible. It is only below the -3dB limit that the effect of amplifiers current noise really increases. In the intermediate region one even has the benefit of less current noise of that resistor. So a higher resistor values is improving the SNR ratio at the low end. The lower frequency limit is better set by a later stage or in the digital domain and the input RC slower by something like a factor of 10.  The downside of a very large resistor is, that it takes longer for the input to settle.
However such low frequency noise measurement will not be very fast anyway - for most sources it takes at least minutes to thermally settle.

With 2200 µF and 1 K the input RC start to contribute to the 0.1 Hz lower limit - so a larger resistor is a good idea, I would even prefer more like 5 K.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 04, 2017, 09:26:42 pm
What is not in the simulation schematic:
I have 2 * 100nF and 2*1000uF (the more leaking) for decoupling.
Between pre-amp and 2nd stage there is also a 100 Ohms in +/- power supply.
At the input there is a additionally 3K6 resistor (with shorting switch for measurement)
to charge up the input capacitor when connecting to a LTZ1000.

I've seen also a couple of diodes (they seems to go only on the LT1012).
They are for reverse voltage protection or what?

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 04, 2017, 09:33:08 pm
Hello,

Yes only for reverse voltage protection.
They are in parallel with the battery. (1N4007)
They short the 9V battery in case of wrong polarity.
Usually they should not be necessary.

By the way the full schematic is attached here (as filt1105w.pdf).

http://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg834013/#msg834013 (http://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg834013/#msg834013)

With best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 12, 2017, 12:50:53 am
So I finished assembly of my unit ... I made initial tests and it seems to works.

I used 120 ohm resistor instead of 100 for the first op-amp gain divider (did't find one 100 ohm in
my stock). And 100K on the second op-amp. This should give me 8K ~ of gain until I do not
replace those resistor with the correct value.

I feed a 1uV and 1mV sine wave to the amplifier at 15 Hz and it give me a 3.6K time amplification
factor that seems to me ok.

Unfortunately I cannot attach any LTZ1000 has they don't like to drive this 2mF cap and start
oscillating. The only buffered LTZ1000 don't feet inside metallic box and if I leave it outside the
box I get all sort of 50Hz crap.

I tested also with batteries and considering the 8K multiplication factor noise floor is very low ...

I will build a buffer to put between the preamp and the LTZ1000 ...

Anyway settling time is almost immediate (max 1 minute), but this was a very luky cap. I leaved
it at 7.3V for 10 days and I found it at this same voltage, leakage is almost zero.

Front:
(https://uploads.tapatalk-cdn.com/20170311/0afd63df0fa435c76e50dd33e22a4730.jpg)

Messy - Back:
(https://uploads.tapatalk-cdn.com/20170311/27b5d1ec349a48302f0ee38ff946006d.jpg)

1uV RMS - 15Hz
(https://uploads.tapatalk-cdn.com/20170311/581e25aebd868b180463735a7868d839.jpg)

4AA alkaline battery noise
(https://uploads.tapatalk-cdn.com/20170311/02a210d4c97757a76bce0f2d75af438d.jpg)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on March 12, 2017, 01:35:23 am
Connect LTZ output thru 3-5k resistor to precharge preamp, and after it's charged (can take hours), short the resistor without breaking the connection.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 12, 2017, 02:29:30 am
Unfortunately I cannot attach any LTZ1000 has they don't like to drive this 2mF cap and start
oscillating.

Hello,

if you have C5 (2,2uF) populated this one could be the reason for the oscillation.
It has no real function and is only left there for EMI reasons. -> feel free to remove the 2.2uF.
But if you put all into a shielded box and supply by battery there should be no EMI.

In any case if connecting to a unbuffered LTZ you have to pre-charge the 2200uF by a  >= 3K3 resistor.
Otherwise you will likely change the LTZ output by overheating. (hysteresis).

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 12, 2017, 07:11:35 am
So I made another test having care to wait 5 minutes before to apply the jumper (that short the 3.6k resistors at input) and this is what I get. Although noise seems very low 6mV p2p at 8000x are 0.75uV.

I have to check what is real gain at 1Hz, I think this is not 8000x as calculated (or maybe this a very good LTZ1000).

This is a screen shot of what I get now.

(https://uploads.tapatalk-cdn.com/20170311/ce08dca5e9a044cc03e65ea0fb683e2f.jpg)

PS: what happens when I run out of batteries with this circuit?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 12, 2017, 07:54:32 am
Hello,

noise seems a bit low for a LTZ1000.
I always had something between 1 and 1.4 uV.
(the 1 uV more for quiet devices with 10 s record length and the 1.4uV more for noisier devices with 100 s record length).

What is your sample rate?
The horizontal resolution looks relatively low for me for a 10s diagram.
(those single pixel width spikes look somewhat unnatural for a 10Hz bw signal if you compare it with my analog screen shots).

Could also be too much scope noise. -> Is the 20 MHz BW-limiter on?
Or is there some averaging active on the scope?
Or is it due to the screen resolution?

Running out of batteries is something that I never tried with a LTZ1000 as DUT.

With best regards

Andreas

PS: the more I think about the screen shot the more I think that the lower frequencies (0.1-1 Hz) are missing.
Are all 6.8uF capacitors really connected?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 13, 2017, 07:46:34 am
Hi Andreas as I stated in previous messages I used only one cap of 2.2mF.

After reading your post I measured bandwidth at 0.1 Hz and signal was less than 50% than at 1 Hz. So this can explain measurement with this "low" value cap.

I'm now testing with a 6.4mF and now noise seems a reasonable amount (8.8mV p2p / 8400 = 1.04uV). Also waveform shape seems to not contain too much high frequency noise.

I still have some high frequency noise coming from the amplifier stage (or caps). This noise have a great improvement if I turn off all led light in the office or if I run all on batteries, but I think it's still there and visible in the chart.

(https://uploads.tapatalk-cdn.com/20170312/378edcd173fdc2522668c4c35589174f.jpg)

As for the scope, bw limit on doesn't make any difference. As sample rate I have to check as in this scope in not shown anywhere (also did a rapid search on the manual but cannot find any paragraph relative to this but they declare up to 1GSPS). No average, no peak detection, just normal sampling. I think this oscilloscope has 640x480 lcd display ... so resolution is just this.

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Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 14, 2017, 06:30:58 am
Hello,

now the results look plausible to me.

Never thought that the input capacitor has such a large influence.
Did you measure the 2200uF capacity value?

With best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 14, 2017, 09:08:47 pm
Andras it was 2.1mF measured at 100Hz.
Anyway you already had -3dB at 0.1Hz with 3mF in your simulation ... not much unexpected.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 16, 2017, 07:56:11 am
Ok guys 23 hours passed from last post in this metrology section, time to break the silence ...

So I cleaned the board, tinned some tracks, removed the 2uF caps at input and that's the result. Pretty nice and all this high frequency noise is gone.

Multiplication factor was precisely determined to be 8720 ... so this 12 mV p2p become 1.4uV p2p in 12 seconds span.

Considering the thingy should have some noise by it self it all seems to work now.

Anyway it's not that practical. All the stuff should be put in a panettone metallic box (biscotti box was too small) including reference and all the batteries.

Any suggestions on how to improve it? It would be good if It can at least run on standard linear power supply.
It seems with this 2uF cap noise is better, but as mentioned before plain LTZ1000 circuit don't like it.
What can you suggest to try for improving it?

(https://uploads.tapatalk-cdn.com/20170315/f855f94efea07a6e3928c5a46333a042.jpg)


Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 16, 2017, 09:36:39 am
Hello,

what are you complaining about?
A set of 9V NiMH batteries will serve you more than a working day for measurements for the amplifier.

Even the Fluke 732B has full specified accuracy only when battery supplied.
So why do you want a extra source of uncertainity?

The only thing is to find a suitable cookies box.
Note that I have a extra metallic box within the cookies box for the amplifier.

You could try a small R/C instead of the 2.2uF to keep some RF noise away from the input of the amplifier.
Perhaps R around 100 Ohms (should keep the LTZ calm, but not increase the current noise too much)
 and C some nF -> far enough away from the upper 10 Hz  bandwidth frequency.

with best regards

Andreas


Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 16, 2017, 11:01:47 pm
Hello,

what are you complaining about?
A set of 9V NiMH batteries will serve you more than a working day for measurements for the amplifier.

Even the Fluke 732B has full specified accuracy only when battery supplied.
So why do you want a extra source of uncertainity?

The only thing is to find a suitable cookies box.
Note that I have a extra metallic box within the cookies box for the amplifier.

You could try a small R/C instead of the 2.2uF to keep some RF noise away from the input of the amplifier.
Perhaps R around 100 Ohms (should keep the LTZ calm, but not increase the current noise too much)
 and C some nF -> far enough away from the upper 10 Hz  bandwidth frequency.

with best regards

Andreas
Don't get me wrong Andreas. Your design is fantastic and just the fact that I was able to get those results for me is astonishing and prove how good is it.

What I'm trying to learn now is how to minimise all those susceptibility effects on the stuff I'm dealing now (that's LTZ1000 circuits and this pre-amp).

I know running on batteries is the best technical solution to get the best from this circuits, but I'm also trying to understand how all this stuff influence the thingies and how to gain a certain degree of immunity.



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Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on March 17, 2017, 03:29:46 am
For the noise meter without the 2 µF to GND, it might be worth adding something to block RF signals. This could be a choke / ferrite bead and maybe 100 pF-1nF to GND. With a 1 nF capacitor one might already want a series resistor in the 50-100 Ohms range to keep sensitive sources (like the LTZ1000 circuit) happy.

I would guess that a lot of spurious signals today are coming from RF signal, like those from mobile phones. So a proper metal case is a good idea.

Another important point is that an OPs output should not be directly connected to an output - it usually needs some kind of output series resistor.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 17, 2017, 03:36:15 am
@Kleinstain

Actually I cannot even take a photo of the scope with my phone. The last photo I published was taken from 2 meters away or the all setup was disturbed.

I'll try to play with some of this stuff in next days.

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Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 18, 2017, 02:07:34 am
When I look at your photo and compare:

On my PCB I have intentionally used no socket for the pre-amplifier stage.
(On the 2nd stage I also have only a socket because I had to exchange the OP-Amp against another type).

I also use some cloth (cotton pads) to keep air currents away from the pre-amplifier on both sides of the PCB.

And finally I keep the input capacitor under bias to keep the leakage current (and according noise) low.
(otherwise you will have to wait 2 days before the noise level settles down each time you want to measure).

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 19, 2017, 08:57:03 am
Hi Adreas,

I found that main source of all my noise problems was the flaky BNC cable I used in first place.
As soon as I replaced with another cheap cable of the same type noise and susceptibility went down and
all measurements now seems quite consistent and reproducible. I can now even use it with "mains" powered
references running out of the panettone box. In this configuration it still pick up some 50Hz common mode
noise, but it's well down to the noise level and really acceptable for "quick" and handy tests setup.

Today, after I swapped the BNC cable I take the chance also to measure noise floor again by connecting a 9V
alkaline battery. After waiting many hours it went down to the equivalent of 150nV (p2p in 12 seconds) that
should be what we want here. If I'm not wrong this should add up a +1% circa of noise when measuring our
LTZ1000 circuits.

>> I also use some cloth (cotton pads) to keep air currents away from the pre-amplifier on both sides of the PCB.

I have also insulated the PCB from the the rest of the box by putting it inside a tight cartoon box.
I didn't put any cotton around the cap, this single 6mF cap is huge so it wan't be effected too much by the small air
draft that can be generated inside the small cartoon box.

I still have some "deflections" now and than, but I think this is mainly because of the cap leakage than because of
thermal drafts. I designed my PCB with slots so that you can thermally insulate the first amplification stage and solder
on some shielding too. In next days (or when the final PCB come) I will try to check what improvement I will have with
this other level of shielding.

>> On my PCB I have intentionally used no socket for the pre-amplifier stage.

I knew this could be a source of problems, but I considered that for a low frequency AC application this should
be less problematic (or not?). I used socket also because I could never imagine that this first prototype will end up
working so well. I will have the PCB fabricated very soon with dual layer layout and my intentions are to not to use
sockets there.

>> And finally I keep the input capacitor under bias to keep the leakage current (and according noise) low.
>> (otherwise you will have to wait 2 days before the noise level settles down each time you want to measure).

Well yes I understood this very soon. My settling time seems to be 8 to 12 hours if I make the cap discharge
for a short period of time, or up to 24 hours if disharge time is longer. More than 24h seems not to improve the
noise.

I also figured out that you can use the open 3.6K ohm resistor jumper pins as shunt to check when leakage
and dielectric absorption has calmed down (be careful if you use it with unbuffered/uncompensated LTZ1000
circuits as it can start oscillating).

So, even if maybe by selecting a less leaky cap and by putting some more thermal/EMI shield overall performance
can be improved I'm now totally satisfied with it. This also confirm how good is your design.

Time to dedicate my time back to the 4 way LTZ1000 averaged circuit. I still have to start building the mini
Arduino programmable thermal chamber too.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 19, 2017, 09:28:34 pm
Good results :-+

by the way: for me it was the 5th design within several months
until I got to my LTZ1000-target  (< 0.3uVpp). I.E. to have
less than 10% influence by the noise floor.

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 19, 2017, 09:51:25 pm
Andreas it was the third PCB I routed and got fabricated. This was the main part I didn't trust. 😁

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Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: pmcouto on March 21, 2017, 01:12:59 am
Hi Andreas and mimmus78,

I’m about to join the club and build this LF-LN amplifier.

Looking at the pictures, I can see you both used WIMA film capacitors.
What type did you use, PET or PP (MKS or MKP)?

Now I’m starting to look for a suitable input capacitor.
Like you, I’m not willing to spend thousands of Euros buying wet slug tantalum capacitors…
Searching my own stock, I could only find some top-brand (Panasonic, Rubycon, CDE) capacitors – all low ESR and rated for 105 C…

Just out of curiosity, I picked a 2,200uF 35V Panasonic (FR series) and checked it for leakage.
As expected, leakage is not as low as required for this project but surprisingly good – The screenshot below shows the results after approximately 10 hours at 10V.

I’ll order some 85 C capacitors from several sources and check them. Hopefully, I’ll find a few meeting the requirements.

Pedro Couto
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on March 21, 2017, 01:43:49 am
Pedro,

I used MKS but I don't know if MKP would be better for this application. I leave the reply to the ones with more knowledge.

I tested all my capacitor stock, and the best one was a single cap rated 6.XmF 16V 105°C, so don't buy more capacitors unless you are sure you tested all your stock. You can find a good capacitor also among 105°C and lower voltage rated ones.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on March 21, 2017, 03:11:59 am
For the film caps MKS should be good enough. The later stages don't contribute significant to noise anyway. MKP might be better in some aspects, but much larger form factor and more expensive at essentially no effect.

For the electrolytic caps it might be worth looking at classical (not low ESR) caps. It could also help (is faster) to do forming at a higher voltage.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 21, 2017, 07:26:27 am

What type did you use, PET or PP (MKS or MKP)?

Now I’m starting to look for a suitable input capacitor.


Hello,

I used WIMA MKS-2. For MKP you would need a monster large cookies box.  ;D

As Kleinstein already mentioned: the most critical is the input capacitor.
Branadic examined that 85 deg C are usually lower leakage than 105 deg C types.
I took what I had in the drawer. (also 85 deg C types).
From 10 good quality capacitors you should be able to get 2-4 suitable ones.
(after 2 days forming).
Solder with minimum heating of the capacitor. Otherwise leakage will rise.

with best regards

Andreas

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: pmcouto on March 21, 2017, 08:36:01 am
Thanks for your very informative replies.

Considering this specific application, I also thought that using MKP vs. MKS capacitors would not result in any measurable difference.
But it’s always better to have other opinions, to make sure one is not missing any important point.  :)

Good to know about your experience selecting the input electrolytic capacitor – You gave me some very useful tips.
I’ve ordered some different capacitors, both 85 C and non “low ESR”. I’ll also measure the leakage current of the ones I currently have in my stock.
Unfortunately I‘m not (yet) equipped to measure several capacitors simultaneously, so this will take a lot of time…

P.S.
I’ll be using MKS caps but that won’t stop me from getting a large cookies box.  >:D


Pedro Couto
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: pmcouto on April 07, 2017, 03:57:46 am
Gerber files sent to PCB house.
PCB was designed to fit inside an aluminum Hammond enclosure (1590T).
Searching for suitable input capacitors is work in progress…  :-DMM

Pedro Couto
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on April 07, 2017, 04:17:20 am
Nice!

Hope to send gerbers of "my" design tomorrow too along with other two others PCBs.

What eda you used?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on April 07, 2017, 04:39:58 am
Gerber files sent to PCB house.
PCB was designed to fit inside an aluminum Hammond enclosure (1590T).

nice design  :-+

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: pmcouto on April 07, 2017, 05:44:32 am
Thanks  :)

I use Altium Designer.


Pedro Couto
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: pmcouto on May 21, 2017, 09:28:09 pm
After some delay in customs, PCBs have finally arrived.
Not perfect, but good enough and acceptable:

(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=317349;image)

PCBs populated with all components, except the input capacitors.
A few words about components used:
-All resistors are 1% or 0.1% metal film
-0.1% resistors are Vishay MPR24 or Welwyn RC55
-All film capacitors are WIMA MKS2
-ICs soldered directly to PCB; no sockets used

(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=317351;image)

Input capacitors were selected from a lot of 70 pieces from different manufacturers (Rubycon PK series, Wurth Elek. WCAP-ATG8 series, ELNA RE3 and RA3 series) and sources such as Farnell, Mouser and eBay (yes, I know that I probably bought some fake capacitors from eBay sellers…).
Capacitors were connected to a PSU set to 80% of nominal voltage and left “soaking” for approximately 3 days.
Then, they were individually measured for leakage using a Keithley 2450 SMU set to source 10V and measure current. After 12 hours, the leakage current was noted.
From tested lot, Rubycon PK series yielded the best results.

Selected capacitors for PCB #1:
Rubycon 35PK2200MEFC16X25 - 2,220uF 35V (Measured 1,859uF@120Hz) – Leakage 5.7nA@10V after 12h
Rubycon 35PK1000MEFCT810X20 - 1,000uF 35V (Measured 833uF@120Hz) – Leakage 3.8nA@10V after 12h

PCB #2:
Rubycon 35PK2200MEFC16X25 - 2,220uF 35V (Measured 1,840uF@120Hz) – Leakage 5.5nA@10V after 12h
Rubycon 35PK1000MEFCT810X20 - 1,000uF 35V (Measured 839uF@120Hz) – Leakage 3.9nA@10V after 12h

I decided to shield the capacitors to reduce EMI. This is probably not needed as the whole PCB is already shielded inside a grounded metal enclosure.
Capacitors were insulated with a couple of layers of Kapton tape and then wrapped in copper tape.
When assembled on the PCB, a wire loop “presses” the capacitor against the large exposed copper area in the PCB, connecting the shield to circuit ground.

(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=317353;image)

Finally, capacitor pins were soldered to the PCB while clamped by an hemostat, to prevent heat from reaching capacitor’s body.
This is fully assembled PCB, ready to go inside the enclosure:

(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=317355;image)

A fully assembled amplifier, ready for testing.
The very crude front panel is just a laminated piece of plain paper, printed on a laser printer.

(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=317357;image)


Next steps:
-Electrical test
-Performance verification


Pedro Couto
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on May 21, 2017, 11:43:34 pm
Hello,

thanks for the tip with the Rubycon capacitors.
seeing forward for your measurements of noise floor.
(Preferably with 8* NiMH AA cells).

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: SeanB on May 22, 2017, 12:45:41 am
Like the label, I did that many years ago, using a copier that had colour cartridges, so I could do a 3 colour print, though I was limited to the available colours of black, blue and red. Then ran it through a hot laminator and cut it out. Nice work with the borders there though, cutting the edges to fit the case so nicely.

For the capacitors, you probably could use 63V capacitors instead of 35V ones, the thicker oxide has lower leakage current at 9V. 3 days of forming probably was excessive, you probably could get away with overnight and then place in a desiccant filled box for a further day, and then measure to find those with highest open circuit voltage, as a proxy for leakage. Tip for reforming is to use the surge voltage rating, via a series resistor, will heal any imperfections and reduce current in normal operation. Slightly reduced lifetime, but thicker oxide all over, as after all they are formed at higher than this voltage.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: pmcouto on May 23, 2017, 07:15:51 pm
Hello,

thanks for the tip with the Rubycon capacitors.
seeing forward for your measurements of noise floor.

These Rubycon capacitors were bought from Farnell, order codes 2346523 (2,200uF) and 2346522 (1,000uF).
Hopefully, I’ll have some time to complete the tests during this week and publish the results.


For the capacitors, you probably could use 63V capacitors instead of 35V ones, the thicker oxide has lower leakage current at 9V. 3 days of forming probably was excessive, you probably could get away with overnight and then place in a desiccant filled box for a further day, and then measure to find those with highest open circuit voltage, as a proxy for leakage. Tip for reforming is to use the surge voltage rating, via a series resistor, will heal any imperfections and reduce current in normal operation. Slightly reduced lifetime, but thicker oxide all over, as after all they are formed at higher than this voltage.

Thanks for the tip regarding capacitor plate (re)forming.
Always learning something new!  :)


Pedro Couto
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: pmcouto on May 28, 2017, 01:38:34 am
I’ve assembled and tested two units. Both units produced almost the same results, so there’s no point in posting duplicate information.
Unit #2 test results below.

Electrical test

-DC voltages checked at OpAmp supply, input and output pins – All OK.

-Current draw is approximately 3mA at each power rail – Within expected value range for this circuit.

-Each amplifier stage checked for correct operation and DC gain accuracy by injecting a DC signal at stage’s input and measuring level at output – Both amplifier stages behaved as expected and measured DC gain was within expected tolerance.


Performance verification

Performance verification tests focused the following characteristics:
-Noise floor
-Filter performance and accuracy
-Gain accuracy

Equipment used:
-Tektronix MDO3054 oscilloscope
-Siglent SDG2122X Arbitrary Waveform Generator
-Mini-Circuits VAT-15 attenuator (x2)
-Huber+Suhner 6630_SMA-50-2 attenuator
-50 Ohm BNC feed through terminator
-BNC-SMA adapter (x2)
-BNC-F to BNC-F adapter
-Non-shorting BNC shielding cap
-BNC-BNC cable (x2)


Noise floor

MDO3054 noise floor, including BNC cable.
One end of the cable connected to scope’s input and the other end shielded by a non-sorting BNC cap.
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=319094;image)

MDO3054+Amplifier noise floor.
One end of the cable connected to scope’s input and the other end connected to amplifier’s output. Amplifier’s input shielded by a non-shorting BNC cap.
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=319096;image)


Filter performance and accuracy and Gain accuracy

A SDG2122X AWG was used to generate a sine wave with an amplitude of 10mVpp. The signal was then attenuated by 60db (/1,000) and fed to amplifier’s input, via a feed through 50 Ohm terminator.
Amplifier’s output was connected to scope’s input, set for DC coupling, 1 MOhm and 20 MHz BW limit. Scope acquisition set to “sample mode” (no signal averaging).

Test setup simplified diagram:
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=319098;image)


Considering expected amplifier’s 80db gain over the BW, oscilloscope should display a signal between 70.7mVpp (-3db point) and 100mVpp for input signals in the 0.1-10Hz frequency range; Outside this frequency range output signal’s amplitude should be below 70.7mVpp.


Lower corner (90mHz – 100mHz)
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=319100;image)

(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=319102;image)


1Hz – 4Hz – 7Hz
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=319104;image)

(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=319106;image)

(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=319108;image)


Upper corner (10Hz – 11Hz)
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=319110;image)


(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=319112;image)



Next steps:
-Post project documentation
-Post some “real world” noise measurements of a voltage reference

Pedro Couto
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on May 28, 2017, 04:37:29 am
MDO3054+Amplifier noise floor.
One end of the cable connected to scope’s input and the other end connected to amplifier’s output. Amplifier’s input shielded by a non-shorting BNC cap.

Hello,

since the input capacitor has leakage current at nominal input voltage this contributes also to the noise floor.
So the more interesting measurement would be with a ultra low noise 10 V source.
(I use 8 NiMh AA cells e.g. ENELOOP @room temperature for it).

Can you further limit the bandwidth on your scope (e.g. to 1 kHz) to reduce the scope noise?
Otherwise it would make sense to increase the gain of the amplifier.
(you want at least factor 3 less noise on the scope than the noise floor of the filter amplifier).
At the moment it looks that both contribute the same amount of noise.

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on May 28, 2017, 06:19:15 am

MDO3054 noise floor, including BNC cable.
One end of the cable connected to scope’s input and the other end shielded by a non-sorting BNC cap.


Mhm,

the output impedance of the amplifier is 150kOhm at 0 Hz and gets lower at higher frequencies.
The scope has 1 Meg.
perhaps it is sufficient to use the unpowered amplifier instead of open BNC to reduce the noise level on the scope.

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: pmcouto on May 28, 2017, 07:44:14 am
the output impedance of the amplifier is 150kOhm at 0 Hz and gets lower at higher frequencies.
The scope has 1 Meg.
perhaps it is sufficient to use the unpowered amplifier instead of open BNC to reduce the noise level on the scope.

I tried that, but there’s no visible difference.
In fact, noise floor doesn’t change even when the input is shorted right at the scope’s input.
It seems that this scope’s noise floor is approximately 680uVpp…


I’ve tried a different scope, a MICSIG TO1104.
This scope allows user setting of BW limiting LP filter. Lowest possible frequency is 30KHz.
Vertical scale can also be set to 500uV/div, allowing better resolution on low-level signals.

The first picture below shows scope’s noise floor, BW limited at 30 KHz.
Scope connected to amplifier’s output, amplifier powered off.

The second picture shows scope + amplifier noise floor, amplifier powered on and input shielded with non-shorting BNC cap.

I have no low noise voltage source or batteries available at the moment; Will test as soon as possible.


Regards,
Pedro Couto
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on May 28, 2017, 02:11:32 pm
Hello,

so with the MICSIG you gain 3 dB on the noise floor.

But obviously the display of the RMS value seems to be the value before bandwidth limiting.
(is obviously too high as it should be around 0.16-0.2 of the peak-peak value with gaussian noise).

With best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: pmcouto on June 11, 2017, 01:31:43 am
Sorry for the delay; My daytime job is keeping me quite busy these days…

Pictures below show noise measurement results of 8xNiMH and 2xLiIon batteries (approximately 10.8V and 8.4V).
Batteries were placed inside a metal box and terminals connected to a panel BNC jack. “Battery Box” connected to Amplifier using a short (20cm) BNC-BNC cable.

The NiMH batteries I could find in my lab were very old (~10 years) and the terminals were tarnished, showing signs of corrosion. Although I was able to bring the cells back to life (well, sort of…) and clean the terminals using a soft wire brush, I don’t trust them – Cell degradation may be contributing to measured noise.
I’ve ordered some new Eneloop batteries and will repeat the test when I receive them.


Project files attached:

SCH – Schematics (numbers in green are Farnell order codes)
PCB – Gerber and Drill files
DG – Drill guide 1:1 (for Hammond 1590T)
FP – Front panel artwork 1:1 (for Hammond 1590T)

Note
Depending on switch manufacturer and model, SW1 and SW2 labels “Measure”/”Charge” and “BIAS”/”ON” on PCB silkscreen and front panel artwork may be reversed.


Regards,
Pedro Couto
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on October 31, 2017, 07:59:17 am
I finally decided to reprint my PCB of the Andreas based noise amplifier along with other PCB I made and today I completed the assembly.

As expected it worked.

I enclose here photo of the assembled PCB and a sample of LM399 and KX LTZ1000 reference.

LM399 is very noiseless track as it was running on batteries.

KX has a little bit of noise as its range is the lowest of my scope and it was also running on mains powered power supply.

I have a couple of more PCBs that I will be happy to give away just shipping costs ... so if you want just ask me.

PS: before someone make me notice ... I know there is italienglish text on the pcb.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on October 31, 2017, 08:33:21 am
Anyway what's best way to calibrate it?

I bodged a 1000 divider with a 10K ohm resistor over a 10 ohm resistor.
The divider output was connected at channel A of the scope and the not divided output of the signal generator was connected to channel B.
I than used a function generator set to 5V DC offset, 100mV sine at 1Hz and trimmed last stage opamp gain to get the 10.000 factor.

I know this is equal to a 10K ohm source ... but considering that gain at 1Hz and at 5Hz is the same, it should be enough time for the capacitor to settle.

What I must do if I'd like to test the preamp at more higher frequency? Buffer the divider output?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on October 31, 2017, 09:10:44 am
Hello,

Don´t understand: a 10K to 10R divider has 10R (ok 9.99 R) impedance at the output
You can also measure the resistors with a DMM to get more accuracy than the resistor tolerance.

10 or 100 Hz is nothing where I would worry about.
The only thing you have to keep in mind is that the input resistance of the amplifier is 1K.
So with 10R impedance at the input you have 1% loss (-1dB) by the 10R to 1K voltage divider.

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on October 31, 2017, 09:49:46 am
Amplitude calibration is one thing. In the middle range the gain is set by the resistor ratio and thus should be rather close to the nominal value. So there should be not much difference from the nominal value.

The second part to check is the filter frequencies. The noise reading (RMS) also depends on the frequency limits. Usually capacitors have much higher tolerance than resistors. So measuring the frequency response can be as important too.
It should be OK to use something like frequency generator and DSO to take the curve. One could get around the frequency calibration if the frequency band is set behind the amplifier.

If very low noise is measured, one should also know the amplifiers own noise to subtract it. So the first test should be checking the noise the amplifier itself, both with a short and open.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on October 31, 2017, 11:02:42 am
OK Andreas I learn something new about divider impedance today ...

I checked -3dB and it was exactly at 10Hz as by Andreas simulation.
Noise shorted (included scope noise) is less than 100nV rms ... with open leads is a 20% more than with shorted leads.
I measured also 2 lipo cells in series and noise was just a little bit more (maybe something like 20nV) but it's hard to tell.

Hello,

Don´t understand: a 10K to 10R divider has 10R (ok 9.99 R) impedance at the output
You can also measure the resistors with a DMM to get more accuracy than the resistor tolerance.

10 or 100 Hz is nothing where I would worry about.
The only thing you have to keep in mind is that the input resistance of the amplifier is 1K.
So with 10R impedance at the input you have 1% loss (-1dB) by the 10R to 1K voltage divider.

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on October 31, 2017, 05:00:14 pm
If very low noise is measured, one should also know the amplifiers own noise to subtract it. So the first test should be checking the noise the amplifier itself, both with a short and open.

Hello,

unfortunately neither of both measurements is the worst case as the leakage current (under bias) of the input capacitor contributes a large amount of low frequency noise.
I recommend using a low noise battery (e.g. several AA NiMh cells) near the maximum voltage that you want to measure (e.g. 10V) as additional test.

The tolerances of the WIMA capacitors which determine the -3dB frequencies is usually well under 5%.
The 3300uF input capacitor is dimensioned in a way that -20% can be tolerated.

with best regards

Andreas


Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: cape zoloh on November 28, 2017, 09:46:19 pm
Anyone of you got a spare PCB to part with?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on November 28, 2017, 09:50:48 pm
I have still some spare one ... send me a message with your address and I will tell you how much will be shipping. PCB is for free.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: 3roomlab on December 11, 2017, 06:57:08 am
i have been playing around with LTspice hovering around these ULNA schematics intermittently. out of the blue, i remember J diddy P 's circuit on ESR, both sides of the probe could be floating w/o a GND. so in theory, the isolation cap will not be charging? should a differential style be more "instant" or "stable" ?? (see pic)
V1 = 2nV pp, output is nearly 14uV pp

what could i have missed in this? as it appears to be too "simple" for a ULNA?

a side track question, what kind of amplification is this normally called? a Bifet? JFET + PMOS?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on December 11, 2017, 07:34:31 am
The amplifier circuit is running without feedback at the input stage. So the amplification will not be very stable.
The critical part is only the first JFET - due to the gain of the first stage, the second stage (P-MOS) is not critical. For this application, there is no need to have differential amplification - just one side is enough. However this leads to a little more distortion - the differential version reduces distortion and DC drift, but also increases noise. However real life the matching will not be that good - so there will be some drift and distortion.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: 3roomlab on December 13, 2017, 05:03:55 am
The amplifier circuit is running without feedback at the input stage. So the amplification will not be very stable.
The critical part is only the first JFET - due to the gain of the first stage, the second stage (P-MOS) is not critical. For this application, there is no need to have differential amplification - just one side is enough. However this leads to a little more distortion - the differential version reduces distortion and DC drift, but also increases noise. However real life the matching will not be that good - so there will be some drift and distortion.

i tried a feedback resistor between G-AA, i suppose that will do as a stabilizer?
it appears i am not good at this  :-DD . G-AA are in phase and will not create a negative feedback

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on January 08, 2018, 02:17:26 am
I have got few more LNAs stashed from pipelie  :-+
Not sure what "A" index means on 3 of the units.

(https://doc.xdevs.com/doc/lymex_pipelie/ulna-1/ulnas_1.jpg) (http://https: :-+//doc.xdevs.com/doc/lymex_pipelie/ulna-1/ulnas.jpg)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: lukier on January 08, 2018, 02:59:29 am
I have got few more LNAs stashed from pipelie  :-+
Not sure what "A" index means on 3 of the units.

Are you going to sell some perhaps?

If not, I might design and build some myself, I might even have the same enclosure (from Aliexpress). Is this the one based on this schematic by zlymex:
(http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/?action=dlattach;attach=233006;image)

(with just micro USB and some USB -> 1S LiPo charger IC added?)?

Thanks!
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Pipelie on January 08, 2018, 03:01:09 am
I have got few more LNAs stashed from pipelie  :-+
Not sure what "A" index means on 3 of the units.

Thank you!
schematic added
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Pipelie on January 08, 2018, 03:07:42 am
I have got few more LNAs stashed from pipelie  :-+
Not sure what "A" index means on 3 of the units.

Are you going to sell some perhaps?

I'm still have some LNAs. if you want to have it, send me a PM.
Thanks.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: branadic on January 08, 2018, 04:59:24 am
How sensitive is this or any other design shown here in matters of humans hand? The 0.1-10Hz LNA design I copied by Andreas needs a cookie box with good ground connection to the scope, otherwise the amplifier is pretty sensitive to humans body. The aluminium profile case I used is not enough shielding.

-branadic-
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: splin on January 08, 2018, 11:08:00 am
i have been playing around with LTspice hovering around these ULNA schematics intermittently. out of the blue, i remember J diddy P 's circuit on ESR, both sides of the probe could be floating w/o a GND. so in theory, the isolation cap will not be charging? should a differential style be more "instant" or "stable" ?? (see pic)
V1 = 2nV pp, output is nearly 14uV pp

what could i have missed in this? as it appears to be too "simple" for a ULNA?

a side track question, what kind of amplification is this normally called? a Bifet? JFET + PMOS?

Small problem - C2 is the wrong way round - C2 and C9 are supposed to eventually end up charged to 0.5 x V1, assuming identical leakage currents for C9 and C2. (Time constant = 50uF,200Kohm = 100 seconds)

But since C2 will be reverse biased the capacitor leakage currents will likely be very different and hence VC2 <> VC9.

Only one capacitor is needed anyway so it perhaps wouldn't make much difference in reality, except that the noise created by the capacitor leakage currents may be much worse, (or maybe less?) due to the reverse bias.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: kj7e on January 08, 2018, 11:09:15 am
How sensitive is this or any other design shown here in matters of humans hand? The 0.1-10Hz LNA design I copied by Andreas needs a cookie box with good ground connection to the scope, otherwise the amplifier is pretty sensitive to humans body. The aluminium profile case I used is not enough shielding.

-branadic-

I've not had any issues with external static or EMI fields with mine.  I can pick it up and move it around without any noticeable effects.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Pipelie on January 08, 2018, 12:51:16 pm
How sensitive is this or any other design shown here in matters of humans hand? The 0.1-10Hz LNA design I copied by Andreas needs a cookie box with good ground connection to the scope, otherwise the amplifier is pretty sensitive to humans body. The aluminium profile case I used is not enough shielding.

-branadic-
Hi branadic,
I have been through that before, when I built the 1.0 version,  the trick is the 10Hz 4th order LPF, unless your scope have a digital filter work as 10Hz 4th order LPF.

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Pipelie on January 08, 2018, 01:22:08 pm
How sensitive is this or any other design shown here in matters of humans hand? The 0.1-10Hz LNA design I copied by Andreas needs a cookie box with good ground connection to the scope, otherwise the amplifier is pretty sensitive to humans body. The aluminium profile case I used is not enough shielding.

-branadic-

I've not had any issues with external static or EMI fields with mine.  I can pick it up and move it around without any noticeable effects.

the LNA Still SENSITIVE to vibration , due to the Piezoelectric effect of Ceramic capacitor  I used  in output stage. if you drop it or strike the case?you will see waveform on scope jumping around.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on January 09, 2018, 02:54:56 am
At the very low level also the electrolytic cap can react to mechanical stress. So not sure it's the caps at the output that make it vibration sensitive.

The 10 Hz upper limit might be a good idea to get direct 0.1-10 Hz readings to compare to a data-sheet. However if used in combination with an FFT, it might be an advantage if a higher upper (and maybe lower lower) frequency limit is possible too.

Also keep in mind that with just adding an extra 2 nd order low pass behind the existing LP will lower the effective upper limit a little. It would be nice to have the calculated frequency response for the amplifier in the instructions. Giving just a frequency range for a noise measurement is tricky, as the filters are not ideal brick-wall and the type of role of can make a difference. For the lower end one might assume a 1/f type noise and thus could get an effective lower limit that works most of the time. But at 10 Hz, it depends on the noise source - the effective upper limit is different if there is dominant 1/f noise at 10 Hz or not.

With an aluminum case the contact to the case can be tricky. Sometimes aluminum does not make a good contact due to a strong oxide layer.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Pipelie on January 16, 2018, 01:18:25 am
At the very low level also the electrolytic cap can react to mechanical stress. So not sure it's the caps at the output that make it vibration sensitive.

The 10 Hz upper limit might be a good idea to get direct 0.1-10 Hz readings to compare to a data-sheet. However if used in combination with an FFT, it might be an advantage if a higher upper (and maybe lower lower) frequency limit is possible too.

Also keep in mind that with just adding an extra 2 nd order low pass behind the existing LP will lower the effective upper limit a little. It would be nice to have the calculated frequency response for the amplifier in the instructions. Giving just a frequency range for a noise measurement is tricky, as the filters are not ideal brick-wall and the type of role of can make a difference. For the lower end one might assume a 1/f type noise and thus could get an effective lower limit that works most of the time. But at 10 Hz, it depends on the noise source - the effective upper limit is different if there is dominant 1/f noise at 10 Hz or not.

With an aluminum case the contact to the case can be tricky. Sometimes aluminum does not make a good contact due to a strong oxide layer.

I think you are right about the electrolytic capacitor. when I built the 1.0 version which doesn't have 4 order LPF(or ceramic capacitor in the signal path), the LNA is not that sensitive to vibration compared to the later version.

I did consider building an LNA with selectable bandwidth, such as 10Hz LPF,100kHz LPF, 1MHz and wide bandwidth, Maybe this year, I hope so.

and about the aluminum case, I'm aware of the oxide layer, but  I don't worry about it, the oxide layer is very thin, and there are multiple points on the case can make good contact.

here are some results about the frequency response. tested by my friend jam.





Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: MiDi on February 11, 2018, 01:26:14 pm
I finally decided to reprint my PCB of the Andreas based noise amplifier along with other PCB I made and today I completed the assembly.
@mimmus78 Thanks for the PCBs!
@Andreas Thanks for the circuit!

Just build one and here are my first results (with 3,3mF ~2nA leakage, circuit without any enclosure):

-3db @0,09/9,7Hz --> all nice
8,7V 33mF: ~200nVpp (10min) --> nice low noisefloor
0V short: ~200nVpp (10min)
--> there is essentially no relevant difference between low noise voltage source and short at input
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on February 11, 2018, 06:53:08 pm

Just build one and here are my first results (with 3,3mF ~2nA leakage, circuit without any enclosure):

--> there is essentially no relevant difference between low noise voltage source and short at input

Hello,

what manufacturer/type of 3,3 mF capacitor did you use (2 nA is really low: at which voltage?).

the first picture with the "artefact" in division 9+10 makes me think of some interference (perhaps with a switchmode supply).
I would put all into a metal cookies box. (I recommend "LAMBERTZ" http://www.lambertz-shop.de/composition-1605.html (http://www.lambertz-shop.de/composition-1605.html))

with best regards

Andreas



Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: MiDi on February 11, 2018, 08:11:44 pm
what manufacturer/type of 3,3 mF capacitor did you use (2 nA is really low: at which voltage?).

the first picture with the "artefact" in division 9+10 makes me think of some interference (perhaps with a switchmode supply).
I would put all into a metal cookies box.

The caps are Panasonic Series: M Typ: A 85°C 2000h.
Had two 3,3mF 25V for first test, one went down to ~2nA, the other to ~3,5nA @~11,5V.
They were formed @11,5V 48h + ~1week disconnected @20°C and <30%rH.
Perhaps my cheap SMPS with "high ripple" had an effect on that, for rechargeable batteries the positive effect with reflex/pulse charging is well known...
Tested with ADA4530-1 @12V as buffer for 12h with recording of voltage drop.
So the 2nA are before soldering, did not test after.

The scope was set with BW-limit to 20MHz (only option for this Rigol) and hires-mode turned on.
As it was a first test with bodged wires, I do not give a thougt on any residues or interference in the readings.
Sure it will go into metal cookie box with coax-cables and BNC-connectors and repeat the measurements ;D
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on February 12, 2018, 12:22:18 am
what manufacturer/type of 3,3 mF capacitor did you use (2 nA is really low: at which voltage?).

the first picture with the "artefact" in division 9+10 makes me think of some interference (perhaps with a switchmode supply).
I would put all into a metal cookies box.

The caps are Panasonic Series: M Typ: A 85°C 2000h.
Had two 3,3mF 25V for first test, one went down to ~2nA, the other to ~3,5nA @~11,5V.
They were formed @11,5V 48h + ~1week disconnected @20°C and <30%rH.
Perhaps my cheap SMPS with "high ripple" had an effect on that, for rechargeable batteries the positive effect with reflex/pulse charging is well known...
Tested with ADA4530-1 @12V as buffer for 12h with recording of voltage drop.
So the 2nA are before soldering, did not test after.

The scope was set with BW-limit to 20MHz (only option for this Rigol) and hires-mode turned on.
As it was a first test with bodged wires, I do not give a thougt on any residues or interference in the readings.
Sure it will go into metal cookie box with coax-cables and BNC-connectors and repeat the measurements ;D
Midi there is slot around first stage of the amplifier. If you manage to add a shield around it and connect the shield near op-amp GND it  should improve your noise.

Don not connect to the general GND as I understand that shielding works better if connected directly to the 100nf caps near the op-amp.

Inviato dal mio ONEPLUS A5010 utilizzando Tapatalk

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: MiDi on February 17, 2018, 10:27:33 am
Now got this into a cookie box and the results are quite good: noisefloor improved by a factor ~2 down to ~115nVpp/10min (there is no difference between input shorted and 33mF cap @8V)
The noise floor with input shorted and circuit turned off is <50nVpp, scope distributes ~35nVpp.
This means a third of noisefloor is not from circuit - perhaps worth checking with DMM @ <= 0.01 PLCs and accurate to <<0.1mV?

For input I salvaged CAT5 patch cable and put together each 4 from different pairs - this should be quite good for shielding electric and magnetic fields and is flexible.
The shield is directly soldered to input low and cookie box, output BNC is isolated from shield/cookie box to omit ground loop (recommend this article about shielding/guarding (https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwj5wNvzuKvZAhUN3KQKHd_aD6cQFggqMAA&url=http%3A%2F%2Fwww.analog.com%2Fmedia%2Fen%2Ftechnical-documentation%2Fapplication-notes%2F41727248AN_347.pdf&usg=AOvVaw3whB0eJVki2lUv5claPSGm))
The cookie box should be tinplate as used in hf-circuits, as it shields electric and magnetic fields and is solderable (other metals like copper or alu do not shield [LF] magnetic fields quite good).

It's time to get serious, but for now other projects are demanding  :-//
First need to get bigger cookie box for putting measured circuit and noiseamp together and apply holes for the cables...

Here are a couple of measurements, as it turned out a 9V block is really noisy (~4,8µVpp) in comparison with 8x mignon ready to use (~200nVpp):
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: cellularmitosis on March 26, 2018, 07:38:49 am
Hello,

I got my hands on one of Pipelie's LNA's and made some measurements of some LTZ1000's.

I did a more in-depth write-up here: https://github.com/cellularmitosis/logs/blob/master/20180324-ltz1000-1f-noise/README.md

I had a question about loading the LTZ circuit.  When discharged, the 1000uF input capacitor in the LNA appears as a 2k load to the LTZ, which would initially draw 3.5mA (decreasing to 1mA after about 3 seconds).  Is it safe to draw this current from an LTZ?  My intuition says yes, because a zener regulates a load by diverting current away from itself, so there is actually 3.5mA less current flowing through the LTZ during this period.

A variation on that question: if the LTZ nominally has about 4mA flowing through it, would there be any adverse effects of attempting to draw more than 4mA from the circuit?

Edit: initially my noise floor wasn't usable.  using my aluminum dutch oven trick solved that.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 26, 2018, 08:14:23 am
Hello,

my intuition says draw not more than 2mA from a unbuffered LTZ.
Since the current regulation loop will need some time to step up.

By the way: the 2K are not the only path where a current can flow.
(So did you really measure the current?)
There is a 510 Ohms via a diode to UBAT (3.7V) to protect the input of the amplifier.
Fortunately there are no clamping diodes between the inputs of the ADA4528.
(otherwise there would be a addinonal path with around 1K to ground).

I have aged my unbuffered LTZs several times by transient pulses where the static load is below 2 mA.
I always use 3K6 in series while charging the input capacitor of my LNA.

If you want to try it: keep an eye on the heater output voltage (with the scope).
Or alternatively to the temperature setpoint. (carefully).

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: hwj-d on March 26, 2018, 10:52:52 am
@cellular,

don't believe you have such a bunch of LTZ's now.  ;D

Your measurements are very interesting.

Would you plan to measure some buffered ones, maybe TEKO used? Don't have one of this LNA's in the moment.

I see, you took some Genrad tempco measurements with selfmade copper wire compensation resistors with promising results too. Awesome  :-+
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Pipelie on March 26, 2018, 12:21:15 pm
Hello,

I got my hands on one of Pipelie's LNA's and made some measurements of some LTZ1000's.

I did a more in-depth write-up here: https://github.com/cellularmitosis/logs/blob/master/20180324-ltz1000-1f-noise/README.md

I had a question about loading the LTZ circuit.  When discharged, the 1000uF input capacitor in the LNA appears as a 2k load to the LTZ, which would initially draw 3.5mA (decreasing to 1mA after about 3 seconds).  Is it safe to draw this current from an LTZ?  My intuition says yes, because a zener regulates a load by diverting current away from itself, so there is actually 3.5mA less current flowing through the LTZ during this period.

A variation on that question: if the LTZ nominally has about 4mA flowing through it, would there be any adverse effects of attempting to draw more than 4mA from the circuit?

Edit: initially my noise floor wasn't usable.  using my aluminum dutch oven trick solved that.

Thanks for sharing the test result of the noise performance of LTZs.
and BTW, when testing unbuffered LTZ, it's wise to use your power supply to charge the input Capacitor to 7V (or equal to the voltage of LTZ) before you connect the LNA to your unbuffered LTZ.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Pipelie on March 26, 2018, 12:49:54 pm
I had a question about loading the LTZ circuit.  When discharged, the 1000uF input capacitor in the LNA appears as a 2k load to the LTZ, which would initially draw 3.5mA (decreasing to 1mA after about 3 seconds).  Is it safe to draw this current from an LTZ? 

I think you can't draw current from LTZ, but you will draw current from the amplifier ( LT1013). and it won't damage the lt1013 or LTZ,  and you have to wait much longer until LTZ Circuits stabilize from “heavy current output” situation.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: kj7e on March 27, 2018, 04:31:49 am
Ill add a 10v Buffered LTZ1000A to this thread,

0.1x prob setting with 10,000x gain LNA.  So divide the displayed voltage by 1,000.  Here, 1.8uV P-P or 307nV RMS over 14 seconds.
(http://i241.photobucket.com/albums/ff146/wsmc551/Elecronics/SDS00001.png) (http://s241.photobucket.com/user/wsmc551/media/Elecronics/SDS00001.png.html)

Noise floor with the reference powered off, 220nV P-P or 94nV RMS;
(http://i241.photobucket.com/albums/ff146/wsmc551/Elecronics/SDS00002.png) (http://s241.photobucket.com/user/wsmc551/media/Elecronics/SDS00002.png.html)

The setup;
(http://i241.photobucket.com/albums/ff146/wsmc551/Elecronics/6C411D6E-9965-4EED-84B7-B0F8798E7999.jpg) (http://s241.photobucket.com/user/wsmc551/media/Elecronics/6C411D6E-9965-4EED-84B7-B0F8798E7999.jpg.html)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 27, 2018, 06:33:53 am
and it won't damage the lt1013 or LTZ, 

Hello,

I would not sign that.

If you draw too much current out of the unbuffered reference the temperature setpoint goes to infinite.
The heater of the LTZ will cook the reference -> quickly ageing and a much larger drift for the next ~6 months.
I tried this accidently several times on my unbuffered references.

The last events where LTZ#1 by -2ppm at day 860 and
LTZ#2 by -5 ppm at day 1220.

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: cellularmitosis on March 27, 2018, 07:07:05 am
I wonder if we can engineer some diode clamping to limit the max heater set-point?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on March 27, 2018, 07:40:10 am
One could definitely add some protection to the heater. A simple one could use something like a divider on the output and a TL431 to check if the voltage is in a reasonable range before enabling the heater.

Another option would be just to limit the power to the heater - this might limit the lower temperature range a little (but who cares about sub zero environment) and make the warm up a few seconds slower.

Both cases would not be a perfect protection, but it should limit possible damage.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: chuckb on March 27, 2018, 01:45:21 pm
Just a quick note.
With an LTZ1000A device and 15V on the collector of the heater transistor, the chip could heat up 300 deg C above ambient, worst case.  If the heater transistor collector voltage is limited to 8V the worst case temperature rise is approximately 80 deg C above ambient. The actual temperature rise will depend on heater resistance, length of LTZ leads, PCB trace size, local airflow and several other things. A small Zener in series with the heater transistor collector can go a long way to prevent damage to the expensive chip.

The LTZ1000 chip does not get that hot because it has better heat conduction between the chip and the case. 

I usually run a separate power supply for the heater transistor until I'm finished developing a circuit.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Pipelie on March 27, 2018, 02:48:34 pm
Just a quick note.
With an LTZ1000A device and 15V on the collector of the heater transistor, the chip could heat up 300 deg C above ambient, worst case.  If the heater transistor collector voltage is limited to 8V the worst case temperature rise is approximately 80 deg C above ambient. The actual temperature rise will depend on heater resistance, length of LTZ leads, PCB trace size, local airflow and several other things. A small Zener in series with the heater transistor collector can go a long way to prevent damage to the expensive chip.

The LTZ1000 chip does not get that hot because it has better heat conduction between the chip and the case. 

I usually run a separate power supply for the heater transistor until I'm finished developing a circuit.

if the 15V was applied to the output of LTZ long enough, it will destroy the LTZ. that happens on my 4950, did I mention it before?
anyway, the cause of this is the tantalum on 15V power rail short, and the Analog switch that shares the same 15V damage(short between the power supply and channel) because Of that, and the 15V go through the switch and apply to the LTZ's 7V Hi. eventually, the heater is broken and short.

so, before you connect your LTZ to LNA, it's better to measure the voltage of the input Capacitor first, if you aren't sure about that.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: cellularmitosis on March 27, 2018, 03:09:19 pm
A small Zener in series with the heater transistor collector can go a long way to prevent damage to the expensive chip.

turns out adding that little zener to my board was a good idea after all :)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: TiN on March 27, 2018, 03:15:07 pm
CM had huge troubles with that little zener  :P His LTZ build became good thermometer (https://xdevs.com/teckit_test2_cm/) data instead of <0.05ppm/K reference  :popcorn: with the zener in heater power rail.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: hwj-d on March 27, 2018, 04:14:36 pm
CM had huge troubles with that little zener  :P

Do you think, that's not a good idea?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: chuckb on March 27, 2018, 11:38:12 pm
The possible "trouble" the voltage dropping zener could cause is loss of temperature regulation. No damage to anything.

The possible "trouble" with 15V on the heater transistor can be a broken LTZ device.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: chuckb on March 28, 2018, 12:08:43 am
With the non A LTZ1000 chip you probably need to stay with 15V applied to the heater transistor. Even with 160 K/watt thermal resistance and 14V on the heater you will have a maximum of 115 K temperature rise. This should not cause serious damage.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on March 28, 2018, 12:22:55 am
It would be a good idea to have the protection adjustable, so that one can adjust the limit to a little more than actually needed (e.g. assume a 10 C minimum environmental temperature).

The thermal resistance depends on the board and cap and the heater resistor as quite a lot of tolerance.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on March 28, 2018, 05:28:05 am
Hmh,

the discussion reminds me of somewhat like:

Take the blue pain killers when you want to touch the hot end of a 400W soldering iron,
and the white ones to touch the 80W.
I would tell my children: never touch the hot end of a soldering iron.
(and use a buffer instead).

SCNR

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: kj7e on March 28, 2018, 04:07:35 pm

Take the blue pain killers when you want to touch the hot end of a 400W soldering iron,


Those blue pain killers sound awesome.  There are days I could use those.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 06, 2018, 02:55:29 pm
Hello,  I decided to build my own 10K:1 amplifier.   It is based on previous designs shared here on EEVBlog with some minor changes I think will be beneficial.  I will attach the proposed schematic and PCB layout below for comments and suggestions.  I will commit to having the PCBs made in about two weeks, so I have time for revisions.   The finished unit is intended to be 9V battery powered and housed in a cast aluminum enclosure roughly 4.7 x 3.5" and 2" deep.   The inputs will be banana jacks and the output BNC or SMA.   The PCB design accommodates both.  Note:  I managed to find non-polarized electrolytic capacitors for the 1000uF and 2200uF coupling capacitors (Rubicon I think).
 

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: chuckb on April 06, 2018, 03:28:24 pm
I see there is a current limiting resistor R1 and that's great, however if the startup sequence is performed out of order then U1 can be damaged. Back to back diodes across R2 or C5 will help prevent damage to the LT1037 input stage.

Last week I used a K617 to compared the leakage current of 1N4148A diodes with 10V reverse bias and a BAV199 Diode pair with reverse 10V bias. The 1N4148A had 10 na and the BAV199 had 40 fa. So the BAV199 had over 100k times less leakage. They are both 150ma diodes.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on April 06, 2018, 05:52:47 pm
R2 is a rather low value resistor. So the amplifier will have a low input impedance - with some sources this could lead to a reduced amplitude. This includes the case when the R1 for protection is active.  In addition R2 would increasethe noise at around the lower frequency range. 
From the noise perspective it is better to not have the input RC to set the lower frequency limit, but have this filter function at a later stage, after some amplification.

So except for faster settling there is not match advantage of a low value for R2.

For conditioning it might be a good idea to have a way to apply something like a 9 V battery to the input cap via a high value resistor. It might take quite some time under voltage to get the really lowest noise from electrolytic caps. This could be a real problem with bipolar caps though. So I am not sure they are a good idea.

The filter stage around U2 could be higher impedance in some points. So one could get away with lower capacitance for C12-C14. For a good filter a proper 2nd order active filter should give a better response curve than just a bunch of 1. st order RC combinations. The last filter stage directly at the output is also a problem, as the input impedance of the scope or what ever is connected would influence the bandwidth and gain.

The LT1012 (like other slow non AZ OPs) does not need that much decoupling - 100 nF + 100 µF would be already plenty. The extra decoupling caps make sense only if they are really close to the chip . so with the current layout the extra 33 nF and 100 nF caps at U1 do not help and may do more harm than good.

It might help to have access to the output of U1. This could be used for 2 purposes: check the input current / settling and for use as a wider bandwidth  (e.g. 0.02 Hz - 10 kHz) amplifier, e.g. with a scope.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 06, 2018, 08:26:19 pm
R2 is a rather low value resistor. So the amplifier will have a low input impedance - with some sources this could lead to a reduced amplitude. This includes the case when the R1 for protection is active.  In addition R2 would increasethe noise at around the lower frequency range. 
From the noise perspective it is better to not have the input RC to set the lower frequency limit, but have this filter function at a later stage, after some amplification.

So except for faster settling there is not match advantage of a low value for R2.

For conditioning it might be a good idea to have a way to apply something like a 9 V battery to the input cap via a high value resistor. It might take quite some time under voltage to get the really lowest noise from electrolytic caps. This could be a real problem with bipolar caps though. So I am not sure they are a good idea.

The filter stage around U2 could be higher impedance in some points. So one could get away with lower capacitance for C12-C14. For a good filter a proper 2nd order active filter should give a better response curve than just a bunch of 1. st order RC combinations. The last filter stage directly at the output is also a problem, as the input impedance of the scope or what ever is connected would influence the bandwidth and gain.

The LT1012 (like other slow non AZ OPs) does not need that much decoupling - 100 nF + 100 µF would be already plenty. The extra decoupling caps make sense only if they are really close to the chip . so with the current layout the extra 33 nF and 100 nF caps at U1 do not help and may do more harm than good.

It might help to have access to the output of U1. This could be used for 2 purposes: check the input current / settling and for use as a wider bandwidth  (e.g. 0.02 Hz - 10 kHz) amplifier, e.g. with a scope.

After reading your post...I decided to run some SPICE simulations of frequency response and changes in R1 and R2.   Results are shown below.   Based on this I will likely raise R2 to 100K and leave the jumper J3 out for most uses.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on April 06, 2018, 11:00:02 pm
After reading your post...I decided to run some SPICE simulations of frequency response and changes in R1 and R2.   Results are shown below.   Based on this I will likely raise R2 to 100K and leave the jumper J3 out for most uses.

Hello,

I would not believe the theoretical ideas of someone who most probably never built a LNA.
(at least not here in the forum).

It needed me several weeks of my hobby time and 5 designs until I had a LNA that works.
(ok I had no experience in this field).
In the first designs I had some strange noise effects that where only
visible if the noise of the source was above a certain level.

If you increase R2 you will also increase current noise of the LT1037.
So you will definitely need a different OP-Amp with lower input bias current.
See Design Notes DN3, DN6 and DN140 of Linear Technology.

And if you really want to leave the protection (J3) off you also have to think about to increase R4 above 3-4K.
(since input protection diodes effectively short the input to R4).

Note:  I managed to find non-polarized electrolytic capacitors for the 1000uF and 2200uF coupling capacitors (Rubicon I think).

Do not forget to select the input capacitor for low leakage current (with maximum used input voltage active).
I fear that bipolar capacitors will have more leakage current than unipolar standard 85 deg C types.
(But you will shurely measure it and report here).

With best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on April 07, 2018, 02:10:19 am
After reading your post...I decided to run some SPICE simulations of frequency response and changes in R1 and R2.   Results are shown below.   Based on this I will likely raise R2 to 100K and leave the jumper J3 out for most uses.
....
If you increase R2 you will also increase current noise of the LT1037.
So you will definitely need a different OP-Amp with lower input bias current.
See Design Notes DN3, DN6 and DN140 of Linear Technology.

A higher R2 will only increase the noise below the input RC passband. The noise of R2 is effectively shorted with the signal source in the pass band. The current noise generated by R2 matters and this is higher with a smaller resistor. An apparent increase in the effect of OPs current noise is due to higher gain at low frequencies. So one would need a different (e.g. addition stage) filter for the low pass to get an overall comparable response. In the pass band region the input capacitor is what matters when it comes to the current noise of the OP.

Still 100 K for R2 might be a little on the high side, as initial settling will take a considerable time.

For the OP it is not directly the input bias that matters, but the input current noise in asymmetrical mode (current for one input only).  Some data-sheets might neglect the correlated part of the input noise, like it applies to an application with a high resistance at both inputs.  So current  noise spec's have to be taken with a grain of salt, especially with AZ OPs.

With simple OPs current noise correlates with input bias, but OPs like the LT1037 or OP07 have input current compensation and with these OPs input bias can be low despite of current noise. So a low bias does not per se guarantees low current noise. It is only a high bias that can guarantee a high current noise.


Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 07, 2018, 01:15:31 pm
After reading your post...I decided to run some SPICE simulations of frequency response and changes in R1 and R2.   Results are shown below.   Based on this I will likely raise R2 to 100K and leave the jumper J3 out for most uses.
....
If you increase R2 you will also increase current noise of the LT1037.
So you will definitely need a different OP-Amp with lower input bias current.
See Design Notes DN3, DN6 and DN140 of Linear Technology.

A higher R2 will only increase the noise below the input RC passband. The noise of R2 is effectively shorted with the signal source in the pass band. The current noise generated by R2 matters and this is higher with a smaller resistor. An apparent increase in the effect of OPs current noise is due to higher gain at low frequencies. So one would need a different (e.g. addition stage) filter for the low pass to get an overall comparable response. In the pass band region the input capacitor is what matters when it comes to the current noise of the OP.

Still 100 K for R2 might be a little on the high side, as initial settling will take a considerable time.

For the OP it is not directly the input bias that matters, but the input current noise in asymmetrical mode (current for one input only).  Some data-sheets might neglect the correlated part of the input noise, like it applies to an application with a high resistance at both inputs.  So current  noise spec's have to be taken with a grain of salt, especially with AZ OPs.

With simple OPs current noise correlates with input bias, but OPs like the LT1037 or OP07 have input current compensation and with these OPs input bias can be low despite of current noise. So a low bias does not per se guarantees low current noise. It is only a high bias that can guarantee a high current noise.

After reading this exchange I decided to hedge my bets and add an "Input R Selection" header to the circuit.  This gives me three options for input impedance with an easy way to change between them.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 07, 2018, 01:19:24 pm

Note:  I managed to find non-polarized electrolytic capacitors for the 1000uF and 2200uF coupling capacitors (Rubicon I think).

Do not forget to select the input capacitor for low leakage current (with maximum used input voltage active).
I fear that bipolar capacitors will have more leakage current than unipolar standard 85 deg C types.
(But you will shurely measure it and report here).

With best regards

Andreas

I will hedge my bets here as well and measure both bi-polar and polar types.  It will be interesting.   What sort of leakage current is acceptable or normal for this application?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 07, 2018, 03:38:00 pm

Note:  I managed to find non-polarized electrolytic capacitors for the 1000uF and 2200uF coupling capacitors (Rubicon I think).

Do not forget to select the input capacitor for low leakage current (with maximum used input voltage active).
I fear that bipolar capacitors will have more leakage current than unipolar standard 85 deg C types.
(But you will shurely measure it and report here).

With best regards

Andreas

I will hedge my bets here as well and measure both bi-polar and polar types.  It will be interesting.   What sort of leakage current is acceptable or normal for this application?

I got to thinking....always dangerous for me...what if I could eliminate the electrolytic caps altogether?  Since I already had the SPICE model running and a good quality 100uF film cap in stock, I ran the simulations.   Not bad response curve.  So just in case, I modified the PCB layout for this configuration as well. 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on April 07, 2018, 07:41:39 pm
The smaller the input cap, the more important the current noise if the OP gets. It is mainly the size of the cap and not R2 that determines the sensitivity to OPs current noise. So there is the option to use a film cap, if a lower current noise OP is used. Due to the current noise, the LT1037 would not perform well with a 100 µF cap below about 1 Hz. The 1/f part of the current noise together with the impedance of the capacitor going up will give a 1/f² noise contribution that limits the use to very low frequencies or needs the very large cap.

Possible candidates would be  OPA140 (as a JFET OP with surprisingly low LF noise) and an low noise AZ OP like ADA4522.
However these OPs have higher voltage noise. So performance will be a bit different - better at some points (frequencies) but worse at others.
These 2 alternatives where discussed earlier in the thread.

All 3 options should be still lower noise than the usual refs over much of the range.  All three have different advantages: The LT1037 (or similar BJT-OP) based one is best in the > 1 Hz range. The OPA140 (maybe 2 in parallel) performs well in the middle and can get away with less protection and the smallest coupling cap. The AZ OP is best at very low frequencies.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on April 07, 2018, 08:55:59 pm
I will hedge my bets here as well and measure both bi-polar and polar types.  It will be interesting.   What sort of leakage current is acceptable or normal for this application?

Hello,

Linear Technology specifies in AN124 a 5 nA max leakage (appendix B) for a 1K2 input pull down resistor.
Design goal is to measure the 775 nVpp of a LTC6655

I specify < 20 nA for the whole input capacitors and a 1K input pull down resistor.
Since I do not want to spend several hundred dollars for a wet tantalum.
See cirquit simulation here:
http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1148584/#msg1148584 (http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1148584/#msg1148584)

Sometimes I get some low frequency artefacts up to 0.2uV (usually noise floor below 120nV) when the capacitor is under bias.
But for a LTZ1000 measurement this noise floor is acceptable.

So the acceptable leakage current is dependant on input resistor and the level where you want to measure.
(you should have at least factor 3-5 lower noise floor as that what you want to measure).

This low leakage current is usually reached after around 2 days of charging.
So I use a 9V block to keep the input capacitor under bias since when not measuring,
since I do not want to wait 2 days for each measurement.

With best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: chuckb on April 08, 2018, 08:13:08 am
Here are some of my experiences over the last two months.

I have used the Nichicon UKL series of Low Leakage coupling capacitors with good results. I use the 2,200 ufd 35V model and they have less than 15na of leakage at 7V. With a new cap I charge it at it's rated voltage for a few days to make sure the dielectric is fully formed. I have not noticed any noise artifacts with them. At 32V the leakage is 65nA.

From a cold start my system is usable within an hour. A precharge battery would certainly help.

After my limited analysis and testing, I decided to use the ADA4522 Chopper for the 10,000x preamplifier. I parallel 4 amplifiers and I have a noise floor under 70 nVpp (0.1 to 10Hz). I tried several other op-amps before settling on the ADA4522. I actually need a little more gain to stay out of the noise floor of the Digital Scope.

The LT1028A Ultra low voltage noise bipolar, had way too much current noise for the 2,200ufd input capacitor, 600nVpp.
The OP627 Super JFET had way too much voltage noise, 500nVpp.
The OPA189 chopper had twice the voltage noise called out in the data sheet. Must have been current noise causing the 200nVpp.

Hope this helps!
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 08, 2018, 11:33:13 am
Here are some of my experiences over the last two months.

I have used the Nichicon UKL series of Low Leakage coupling capacitors with good results. I use the 2,200 ufd 35V model and they have less than 15na of leakage at 7V. With a new cap I charge it at it's rated voltage for a few days to make sure the dielectric is fully formed. I have not noticed any noise artifacts with them. At 32V the leakage is 65nA.

From a cold start my system is usable within an hour. A precharge battery would certainly help.

After my limited analysis and testing, I decided to use the ADA4522 Chopper for the 10,000x preamplifier. I parallel 4 amplifiers and I have a noise floor under 70 nVpp (0.1 to 10Hz). I tried several other op-amps before settling on the ADA4522. I actually need a little more gain to stay out of the noise floor of the Digital Scope.

The LT1028A Ultra low voltage noise bipolar, had way too much current noise for the 2,200ufd input capacitor, 600nVpp.
The OP627 Super JFET had way too much voltage noise, 500nVpp.
The OPA189 chopper had twice the voltage noise called out in the data sheet. Must have been current noise causing the 200nVpp.

Hope this helps!

Can you share the details of how your parallel the four amplifiers?  Separate feedback loops and some series R perhaps? 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: David Hess on April 08, 2018, 01:31:48 pm
What about a differential input stage using something like an LSK389 dual JFET driving an LT1001 with a chopper stabilized amplifier providing offset and low frequency correction?  Jim Williams showed this type of amplifier in AN124 and some earlier application notes.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: chuckb on April 08, 2018, 01:48:04 pm
Overall my circuit is similar to Pipelie's design. I used his output filter values with Wima plastic leaded caps and R-R op-amps. After the input cap (two 2,200 ufd caps in parallel) and clamping diodes the signal splits into the input of four ADA4522 op-amps. They have a 200k MF (SMD 0805) feedback resistor and a 20 ohm MF to ground. These are all surface mount parts. I also use a leaded COG 0.1ufd cap across each 200k feedback resistor to minimize high frequency noise. The output of each amplifier passes through a 10k resistor to sum the signals together before feeding the output filter stage.

I have not asked Analog Devices yet if the two amplifiers in the ADA4522-2 package are completely independent or if they share a common clock. If they share a clock the input and output noise spikes will be correlated and you will not achieve the theoretical noise reduction from paralleling.  Some dual and quad op-amps share a common bias circuit so a voltage overload on one op amp will affect the others in the same package.

I had a 12v battery handy to power the circuit so I use two diodes to develop a -1.2v bus and a +10.8v bus.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on April 08, 2018, 02:28:07 pm
I actually need a little more gain to stay out of the noise floor of the Digital Scope.
Hello,

Which scope do you use?

Usually you have at least a 20 MHz bandwidth limiter on a scope.
Better scopes have either some kind of oversampling (e.g. hi-res aquisition mode)
or a additional digital filter to further reduce scope noise.
For a 10 Hz bandwidth and enough horizontal sample points you can use
a 1 kHz bandwidth filter on the scope to reduce scope noise floor without spoiling the measurement.

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 08, 2018, 02:53:33 pm
Overall my circuit is similar to Pipelie's design. I used his output filter values with Wima plastic leaded caps and R-R op-amps. After the input cap (two 2,200 ufd caps in parallel) and clamping diodes the signal splits into the input of four ADA4522 op-amps. They have a 200k MF (SMD 0805) feedback resistor and a 20 ohm MF to ground. These are all surface mount parts. I also use a leaded COG 0.1ufd cap across each 200k feedback resistor to minimize high frequency noise. The output of each amplifier passes through a 10k resistor to sum the signals together before feeding the output filter stage.

I have not asked Analog Devices yet if the two amplifiers in the ADA4522-2 package are completely independent or if they share a common clock. If they share a clock the input and output noise spikes will be correlated and you will not achieve the theoretical noise reduction from paralleling.  Some dual and quad op-amps share a common bias circuit so a voltage overload on one op amp will affect the others in the same package.

I had a 12v battery handy to power the circuit so I use two diodes to develop a -1.2v bus and a +10.8v bus.

I submitted a question to AD via their website asking about a common clock on the ADA4522-2 and ADA4522-4 parts.   I will post a reply if I get one.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Pipelie on April 08, 2018, 04:00:32 pm
Overall my circuit is similar to Pipelie's design. I used his output filter values with Wima plastic leaded caps and R-R op-amps. After the input cap (two 2,200 ufd caps in parallel) and clamping diodes the signal splits into the input of four ADA4522 op-amps. They have a 200k MF (SMD 0805) feedback resistor and a 20 ohm MF to ground. These are all surface mount parts. I also use a leaded COG 0.1ufd cap across each 200k feedback resistor to minimize high frequency noise. The output of each amplifier passes through a 10k resistor to sum the signals together before feeding the output filter stage.

I have not asked Analog Devices yet if the two amplifiers in the ADA4522-2 package are completely independent or if they share a common clock. If they share a clock the input and output noise spikes will be correlated and you will not achieve the theoretical noise reduction from paralleling.  Some dual and quad op-amps share a common bias circuit so a voltage overload on one op amp will affect the others in the same package.

I had a 12v battery handy to power the circuit so I use two diodes to develop a -1.2v bus and a +10.8v bus.

archwang from BBS.38HOT.NET successfully to use ada4522-4 in paralleling, and 60nVpp noise floor achieved when input short.
and  100nVpp when the input is open.
here is the photo
(http://bbs.38hot.net/data/attachment/forum/201606/13/205819c9ccbxb5656nee88.jpg.thumb.jpg)
(http://bbs.38hot.net/data/attachment/forum/201606/13/210034jdqmrlfmah6qqlf3.jpg.thumb.jpg)

test setup:
(http://bbs.38hot.net/data/attachment/forum/201606/13/210354v8qpqqlm8h88fkpq.jpg.thumb.jpg)

test results:
1.input short
(http://bbs.38hot.net/data/attachment/forum/201606/13/211046xnpu8sz71x3guxpb.jpg.thumb.jpg)
(http://bbs.38hot.net/data/attachment/forum/201606/13/213513jyxj333dz0ohh3yd.jpg.thumb.jpg)

2.input open:
(http://bbs.38hot.net/data/attachment/forum/201606/13/214152t33mv4vv50a505dv.jpg.thumb.jpg)

3.testing a 9V battery
(http://bbs.38hot.net/data/attachment/forum/201606/13/214320ol5leotv55oqbg2o.jpg.thumb.jpg)

here is the link of  archwang's article.
http://bbs.38hot.net/forum.php?mod=viewthread&tid=164706 (http://bbs.38hot.net/forum.php?mod=viewthread&tid=164706)

Have fun!
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: chuckb on April 09, 2018, 01:29:28 pm
I actually need a little more gain to stay out of the noise floor of the Digital Scope.
Hello,

Which scope do you use?

Usually you have at least a 20 MHz bandwidth limiter on a scope.
Better scopes have either some kind of oversampling (e.g. hi-res aquisition mode)
or a additional digital filter to further reduce scope noise.
For a 10 Hz bandwidth and enough horizontal sample points you can use
a 1 kHz bandwidth filter on the scope to reduce scope noise floor without spoiling the measurement.

with best regards

Andreas

I'm using an old TDS3034, with floppy drives! It has 360-380uVpp noise with the input shorted and BWL at 20MHz. With the full 100 Mhz BW it has 700-800uVpp. That's all the noise filtering this scope has.

I'm working on connecting it to a Dynamic Signal Analyzer (HP3582A) to plot the actual preamp stage gain and the output filter performance. I have to attenuate the tracking generator output before it gets to the preamp. I have lots of 60Hz noise right now. The specs tell me it weights 54 lbs, it feels a lot heavier than that. It's on the corner of the bench and it's staying there.
Take care
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 09, 2018, 08:17:00 pm
Re: 10/100K:1 Amplifier

After absorbing a lot of advice and modifying my original layout many times, I decided to start over.   The schematic was out of sync with the PCB and I really need more space to do what I wanted.  So here is the next and hopefully close to last iteration.   I will be traveling for the next few days, so forgive me if I don't respond until the weekend.  On the schematic, the yellow capacitors have alternate footprints allowing for use of different components.  These are for the largest values only.   The first op-amp can be one of two types.  All circuit values assume use of an ADA4522-4 Quad op-amp for the first amplifier.   The LT1037 shown highlighted in blue, can be used instead.  In that case, R8 is changed to 2.5K and all components associated with the other three ADA4522 channels are not used (HIGHLIGHTED in magenta).  The circuit allows for 10K:1 or 100K:1 via a jumper on the second stage amplifier.   Also variable is the input resistance in three steps, 100K, 16.7K and 990 ohms.   The range of the pot and "boost adj" jumper should allow for calibration in all combinations of J3 and J4.   

On a side note I am also measuring leakage on several types of electrolytic capacitors.  My setup needs improvement....measuring nA isn't easy.   I plan on putting the measurement into a steel enclosure and trying again once the box arrives late next week (hopefully).   I, can measure down to 10's of nA currently, but the noise prevents anything less than that. 

Comments and suggestions are solicited.

Note the schematic has been replaced with the correct version.  An incorrect version was previously posted.

Note this post has been superseded by a later version posted 14 Apr.

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: MiDi on April 09, 2018, 11:20:35 pm
The caps are Panasonic Series: M Typ: A 85°C 2000h.
Had two 3,3mF 25V for first test, one went down to ~2nA, the other to ~3,5nA @~11,5V.
They were formed @11,5V 48h + ~1week disconnected @20°C and <30%rH.
Perhaps my cheap SMPS with "high ripple" had an effect on that, for rechargeable batteries the positive effect with reflex/pulse charging is well known...
Tested with ADA4530-1 @12V as buffer for 12h with recording of voltage drop.
So the 2nA are before soldering, did not test after.

Do not know how others did, but I used an ADA4530-1 as buffer and recorded voltage drop over decent time.
fA op amp is a bit overkill for this purpose, but did this for measuring leakage of film caps in first place.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: chuckb on April 09, 2018, 11:32:04 pm
I determine leakage current by measuring the voltage drop across a 10k charge resistor. Initially the cap is connected directly to the power supply but after a few hours I add a resistor in line. With a cap connected directly to the PS through an ammeter I realized I was really measuring to power supply voltage ripple. The 10k will convert 10na of leakage into a 100uV DC signal. This worked fine for me even using a very noisy 10 amp switching power supply.

In bad cases you could run two of the RC filters in series. One to clean up the PS and the second to actually test the capacitor.

Side note-
I make it a point to not connect a discharged cap directly across a PS at full voltage. Also I don't short out the charged caps, I use a resistor to limit the peak current, the 10k does double duty. I had a custom low ESR Wet Tantalum capacitor developed for one project many years ago. As a test, I measured over 500 amps of charging current into the capacitor. Yes, the sparks are fun but I just worry about developing secondary problems at the internal weld between the lead to foil (for Al Electrolytics).

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on April 10, 2018, 03:27:10 am
For the low pass filtering stages, there are quite a few caps and the values do not all make sense. There are plenty of low pass stages, so there should be no need to have the large caps at the 4 parallel ADA4522 stages. This would also alow to use the first stage only with higher bandwidth if needed.
R20 does not make that much sense with the up to 4 parallel 10 K resistors at the OPs. So likely C24 is too large.

The wiring of the lower two AD4522 is wrong.

There are still the sets of 3 caps for decoupling - this is only needed for super fast parts like 74AC... or OPs in GHz GBW range, not for a slow LT1012. The LT1012 is happy with just a 10 µF electrolytic cap somewhere on the board.  For the caps layout is often more important than a pure number. An extra 33 nF at more than a 1 cm away from the chip does not help anymore an is more like a possible problem.

The layout is kind of a mess. With the very high overall amplification a simple ground plane may not be such a good idea, especially with the decoupling caps spread all over the board.

Using a +-9 V supply may not be the best option, as there is no real need for a high voltage. The ADA4522-4 is perfectly fine with just 4-6 V or so, but it needs quite some current.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 13, 2018, 07:56:01 pm
For the low pass filtering stages, there are quite a few caps and the values do not all make sense. There are plenty of low pass stages, so there should be no need to have the large caps at the 4 parallel ADA4522 stages. This would also alow to use the first stage only with higher bandwidth if needed.
R20 does not make that much sense with the up to 4 parallel 10 K resistors at the OPs. So likely C24 is too large.

The wiring of the lower two AD4522 is wrong.

There are still the sets of 3 caps for decoupling - this is only needed for super fast parts like 74AC... or OPs in GHz GBW range, not for a slow LT1012. The LT1012 is happy with just a 10 µF electrolytic cap somewhere on the board.  For the caps layout is often more important than a pure number. An extra 33 nF at more than a 1 cm away from the chip does not help anymore an is more like a possible problem.

The layout is kind of a mess. With the very high overall amplification a simple ground plane may not be such a good idea, especially with the decoupling caps spread all over the board.

Using a +-9 V supply may not be the best option, as there is no real need for a high voltage. The ADA4522-4 is perfectly fine with just 4-6 V or so, but it needs quite some current.

This circuit is based on a previous successful design previously posted here on EEVBlog.  I'm not sure if Andreas was the first to post this design or not, but i got the basic circuit from his posts.  I have modified this circuit in a variety of ways, among them using a quad op-amp, duplicating the values used in the single op-amp design and summing the outputs.   In the original circuit 3.6K in series was used between op-amp stages.  I wanted to use 10K summing resistors basically because I have a large stock of this common value, this necessitated R20 to make up the difference between the parallel combination of the 4x 10K resistors and the 3.6K net value I wanted to preserve.   I have also added some small value capacitors in the hope to minimize noise.  These can of course simply not be used if experimentation shows them unnecessary or detrimental.   I also realize that component values may not be optimum, but I want to design a PCB that would allow me to return to a design that is known to work, but would also allow for some experimentation, both in component values and additional components where I thought they might be beneficial.

The schematic error was actually caught during the PCB routing process, but unfortunately, I posted the uncorrected schematic in error.  The corrected version is attached and has also been replaced in the original post.

I understand that the value of the bypass caps seems very large for these op-amps, but that was what was used in the original circuit (470uF).  So I wanted to preserve the option in the PCB, thus using large enough pads to accommodate them.   A test with smaller values would be interesting.   I also see you point about using both 100nF and 33nF bypass caps...again I can eliminate one set, giving the option to avoid surface mount components if desired. 

The layout isn't perfect to be sure, but should work with some further modification.  I agree on your observation about the ground plane and my thinking while traveling this week on how to change it to further isolate ground currents from the sensitive input circuitry.  I will post another version of the layout soon.   

Your point on 5V supply voltage is well taken.   The original circuit used 9V, but going with 4x 1.5V AA or AAA cells in series per side might be a better choice.  Again I will experiment with this. 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 13, 2018, 08:05:42 pm
Overall my circuit is similar to Pipelie's design. I used his output filter values with Wima plastic leaded caps and R-R op-amps. After the input cap (two 2,200 ufd caps in parallel) and clamping diodes the signal splits into the input of four ADA4522 op-amps. They have a 200k MF (SMD 0805) feedback resistor and a 20 ohm MF to ground. These are all surface mount parts. I also use a leaded COG 0.1ufd cap across each 200k feedback resistor to minimize high frequency noise. The output of each amplifier passes through a 10k resistor to sum the signals together before feeding the output filter stage.

I have not asked Analog Devices yet if the two amplifiers in the ADA4522-2 package are completely independent or if they share a common clock. If they share a clock the input and output noise spikes will be correlated and you will not achieve the theoretical noise reduction from paralleling.  Some dual and quad op-amps share a common bias circuit so a voltage overload on one op amp will affect the others in the same package.

I had a 12v battery handy to power the circuit so I use two diodes to develop a -1.2v bus and a +10.8v bus.

I submitted a question to AD via their website asking about a common clock on the ADA4522-2 and ADA4522-4 parts.   I will post a reply if I get one.

I got an answer from Analog Devices on this matter:

Hi Insatman (name replaced with my handle here),
It is a common clock but quite high up around 5MHz.
There's also another balancing chopper circuit running around 800kHz.
These are noted in Figure 62.

Thus, you'll probably need an 80kHz LPF. 

Chris

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on April 14, 2018, 12:26:28 am
I understand that the value of the bypass caps seems very large for these op-amps, but that was what was used in the original circuit (470uF).

Hello,

actually I am using 1000uF for the decoupling because I have several left from the leakage current selection process.
Together with the 100R decoupling resistors they give a fine low pass for the supply in the first stage.
The fact that a LT1037 which was previous in the output stage gives oscillations shows that decoupling is not uncritical.

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 14, 2018, 01:18:17 pm
This is the version of the 10/100K:1 amplifier I sent out for fabrication today.
I should have the boards in 5-10 days.

I wish to thank all those who gave me advise and/or criticism...it results in a better design and learning experience for us all.
 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on April 28, 2018, 02:13:18 pm

Do not forget to select the input capacitor for low leakage current (with maximum used input voltage active).
I fear that bipolar capacitors will have more leakage current than unipolar standard 85 deg C types.
(But you will shurely measure it and report here).

With best regards

Andreas

I have done some tests on various Electrolytic capacitors.   11 were tested comprising of 8 individual types.   The data is presented in a table as well as three bar charts. 

Note that the time tested for various caps was not equal.  I tended to end the test when the current was well below 5nA and/or I established a trend in the data upward/downward.  Most tests were started with the caps pre-charged tp 10.0 volts for some hours beforehand.  One test was started at 9.7V which is what my 9V battery had pre-charged the capacitor too before the test.   Another test was started at 0.0V and run overnight.   

I built a steel enclosure and special cables to do these tests.  The enclosure was made by Hoffman and is heavy gauge steel.  All seams were sealed with conductive glue copper tape as well.  Contact surfaces between the lid and box were also copper taped.   The cables were made from RG58 coax with the shields terminated just short of the banana connection points to the power supply or bench meter.  Only one connection between box/shield ground was made at the power supply to avoid any ground loops.   All cables were fitted with ferrite cores as well.  This resulted in an exceptionally quiet setup.   

Leakage current was measured across a metal film 1 Meg ohm resistor.  The meter used was an HP34401A with 1Gohm input impedance selected.  This allowed me to get 1mV per nA on the meter.  Noise floor was estimated to be typically below 100's of pA based on the observed fluctuations of the readings.   

One bar chart shows starting and ending currents by capacitor number (1-11).  Another chart shows the leakage current per uF of capacitance for an easier comparison of types.  the third chart shows the rate of change in the data to get an idea of the average trend when the test was stopped.   Note 2 capacitors had an upward trend in leakage current.  I would reject these two capacitors. 

Overall the Nichicon Gold Audio caps are favored by me at present.  One of the three had upward current trend, but the other two performed very well.   Second choice would be the Nichicon KL series.   The Bi-polar types had very low leakage currents if you soaked them long enough in the direction you intended to use them in.  I think they would also work.

One photo shows the test setup on my messy workbench.  The capacitor being measured at 540nA was fully discharged before the test began.  This test is to get and idea of how long conditioning takes from a cold start.   Note the various caps being pre-charged on the protoboard on top of the steel box used for testing.

I will be leaving on a vacation soon, so If I may be slow to respond to comments depending on when they are made. 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on May 30, 2018, 08:44:24 pm
Back in the middle of April I submitted a design for a 10/100K to one amplifier based on the design that Andreas had previously described.   I included a variety of modifications to that design to hopefully lower the noise level and also too provide some flexibility for experiments with various components.   After much testing I have some results.
I started testing using my Rigol 1054Z but found the noise floor to be excessive at about 1mV pk-pk.  Further work was done on my Tek 784D with pk-pk noise of <200uV.  These values were measured with 20M bandpass, 1M input impedance, DC coupling and open input.

All tests were done at 10000:1  the 100K:1 option was not tested at this time.

Two PCBs were made.  One, dubbed “Orig” is pretty close to the circuit that Andreas described with the addition of some ferrites and small bypass capacitors on the input and outputs.   This version uses the LT1037 for the 1st stage amplifier and an LT1012 for the second stage.   This unit was tested using the original 1K input load resistor that Andreas had and also with an alternative 20K input load resistor with 2K in series with the input.   The Orig circuit performed well with a noise floor of about 1.3mV (equating to 130nV) on average.  The 1K version was very slightly lower noise than the 2k/20K version.  Frequency response was surprising however.  Both the 1K version and 2/20K versions had -3dB points of ~0.1-4Hz.   Settling time for each measurement is long…often taking a minute or so for both op-amps to come out of saturation and settle near the baseline.

The 2nd PCB, dubbed “New” incorporates a surface mount quad op-amp (with all amps in parallel) for the first stage.  The op-amp selected is the ADA4522-4.  This was chosen because of Pipelie’s recent posts.   The 2nd stage amplifier is the same LT1012 circuit used on the Orig circuit.   This new version was first tested using a 100uF film capacitor instead of the 3300uF electrolytic used on the Orig PCB.  The 100uF cap was only tested in the 2/20K mode.   Noise floor was similar to the Orig version with 3300uF capacitor at ~1.3mV (130nV) pk-pk.  Frequency response for this version was ~0.2Hz-4Hz (-3dB).
At this point I decided to address the bandpass and get the high-end to around 10Hz rather than 4Hz.   Doing some SPICE simulations, I zeroed in on the value of some capacitors in the original circuit.  Namely C16, C25, C26 and C27.  All were 220nF.  I changed all four to 47nF (FILM).   Subsequent bandpass tests showed ~0.2Hz to 9Hz (-3dB). 
Next tests involved changing the 100uF capacitor out for another 3300uF Nichicon Gold Audio capacitor.  The same type used in the Orig circuit.  Note these caps were selected for very low leakage current (<5nA at 10VDC).  Of the six caps purchased four met this test but required several hours to overnight 10V bias before getting down to that level.   I did not bias the caps before testing however.  All testing was one without pre-bias being applied.

The New circuit using the 3300uF cap and 1K shunt resistor yielded the best performance.  Noise was typically around 1.2mV pk-pk (120nV),   Frequency response was ~0.1Hz to 11Hz (-3dB). 

Setting the gain for each circuit setup was done by applying a 1Hz sinewave attenuated to ~5mV.  This is directly measured on the scope.  An additional checked 20dB attenuator is then placed in series to give ~500uV signal.   The amplifier gain is then adjusted to get ~5V output (whatever the calculated input signal is x10000).  Then the signal is further attenuated to ~5uV and the resulting signal check for reasonableness.  I found the apparent gain was always larger than expected by 6-13% when doing this check.  A check of my attenuators on the SA showed then not to be the primary source of this error.   
Could it be that gain at very low input voltage is somewhat non-linear?   Even with the apparent gain error, the amplifier is still quite useful, but it does put some uncertainty on amplitude of noise measurements.

Insatman


Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on May 31, 2018, 01:17:31 am
To measure the noise of the amplifier itself, the short circuit case is more relevant than the open case. This will especially lower the noise for the version with higher resistor to ground (e.g. 2 K / 20 K and 100 µF cap). Usually signal sources are kind of low impedance compared to the resistor at the input - if not, one has to include the reduced gain / source loading. So the open circuit test is misleading.

The gain is expected to be linear down to low amplitudes. There are quite a few resistors that reduce the gain a little, but the simulation should tell.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on May 31, 2018, 01:09:26 pm
To measure the noise of the amplifier itself, the short circuit case is more relevant than the open case. This will especially lower the noise for the version with higher resistor to ground (e.g. 2 K / 20 K and 100 µF cap). Usually signal sources are kind of low impedance compared to the resistor at the input - if not, one has to include the reduced gain / source loading. So the open circuit test is misleading.

The gain is expected to be linear down to low amplitudes. There are quite a few resistors that reduce the gain a little, but the simulation should tell.

Amplifier noise was checked with a 50 ohm terminator at the input and the case closed (see JPG on post marked SETUP).  The scope input noise was checked without a terminator. 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on May 31, 2018, 01:41:13 pm
Based on experience from my last PCB design I decided to make a new one.  The original design had a few errors that required bodges and a couple of the component spacings were off a bit.  The new layout is much cleaner and is mechanically compatible with the original PCB design.   Gone is the ability to use a 100uF film capacitor in place of the 3300uF electrolytic, to use smaller multiple film capacitors for the large 22uF film cap or to substitute U1 (ADA4522-4) with LT1037.   Schematic and layout attached.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on May 31, 2018, 02:00:29 pm
Both the 1K version and 2/20K versions had -3dB points of ~0.1-4Hz.   

Settling time each time a measurement is started is long…often taking a minute or so for both op-amps to come out of saturation and settle near the baseline.

probably the additional low pass R8/C16? could be the reason.
R8 (10K) + the following 1K (R20?) resistor are larger than the 3K6 in my cirquit.

It is clear (to me) that the settling time is long for a 0.1 Hz lower frequency corner.
(Time constant 3.3 sec and at least 10-20 Tau to settle to uV level)

To measure the noise of the amplifier itself, the short circuit case is more relevant than the open case.
The relevant case is with a low noise around 10V source (e.g. 8*NiMH AA cells at constant temperature).
Otherwise you do not get the real life noise due to leakage current of the input capacitor.
A short or open will give too low values for the noise floor.

with best regards

Andreas

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on May 31, 2018, 02:12:51 pm
Both the 1K version and 2/20K versions had -3dB points of ~0.1-4Hz.   

Settling time each time a measurement is started is long…often taking a minute or so for both op-amps to come out of saturation and settle near the baseline.

probably the additional low pass R8/C16? could be the reason.
R8 (10K) + the following 1K (R20?) resistor are larger than the 3K6 in my cirquit.

It is clear (to me) that the settling time is long for a 0.1 Hz lower frequency corner.
(Time constant 3.3 sec and at least 10-20 Tau to settle to uV level)

Andreas

In the case of the "Orig" circuit where the LT1037 is used, a 2.5K ohm resistor is used rather than a 10K resistor.  This is noted on the schematic.  R20 is 1.2K, so the combination is 2.5 + 1.2 giving 3.7K.  Quite close to your origional circuit.   What I forgot to mention in my post is that the "New" version had significantly faster settling time, especially with the 100uF cap installed.   I'm guessing that the LT1037 just takes much more time to come out of saturation than the ADA4522-4.   
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on June 01, 2018, 01:01:59 am
Electrolytic caps have a rather long internal settling time due to dielectric absorption. So independent on the capacitance value it is just the capacitor internal that takes a long time to settle. Depending on the capacitor type this can take rather long - well in to the seconds or even minutes for the time constant. Checking the settling / dielectric absorption could be another parameter in addition to leakage to look at when choosing caps.

If it would be just the RC time-constant at the input and a later filter stage, there would be the option to speed things up, but reducing the resistor for the initial settling phase.

The LT1037 should be very fast to come out of saturation (more like < 1 µs range). An AZ OP like the ADA4522 usually takes a little longer (10 µs - a few ms) - especially the modern ones with higher GBW may be a little faster. The difference is more like the difference film cap versus electrolytic cap.

The real world test is with a low noise voltage source - especially with an electrolytic capacitor.  With a film capacitor the extra noise compared to a short should not be much, but with an electrolytic the extra noise due to leakage can be important. The open input could give a higher noise, as the resistor to ground in not shunted paralleled with the signal source. So it depends on the input capacitor if the open input noise is higher of lower than the real test with a voltage source.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on June 01, 2018, 12:02:10 pm

The relevant case is with a low noise around 10V source (e.g. 8*NiMH AA cells at constant temperature).
Otherwise you do not get the real life noise due to leakage current of the input capacitor.
A short or open will give too low values for the noise floor.

with best regards

Andreas

I have ordered some NiMH cells and a charger to do this test.  They are not readily available here, so Digikey to the rescue. 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on June 01, 2018, 12:19:12 pm
Electrolytic caps have a rather long internal settling time due to dielectric absorption. So independent on the capacitance value it is just the capacitor internal that takes a long time to settle. Depending on the capacitor type this can take rather long - well in to the seconds or even minutes for the time constant. Checking the settling / dielectric absorption could be another parameter in addition to leakage to look at when choosing caps.

If it would be just the RC time-constant at the input and a later filter stage, there would be the option to speed things up, but reducing the resistor for the initial settling phase.

The LT1037 should be very fast to come out of saturation (more like < 1 µs range). An AZ OP like the ADA4522 usually takes a little longer (10 µs - a few ms) - especially the modern ones with higher GBW may be a little faster. The difference is more like the difference film cap versus electrolytic cap.

The real world test is with a low noise voltage source - especially with an electrolytic capacitor.  With a film capacitor the extra noise compared to a short should not be much, but with an electrolytic the extra noise due to leakage can be important. The open input could give a higher noise, as the resistor to ground in not shunted paralleled with the signal source. So it depends on the input capacitor if the open input noise is higher of lower than the real test with a voltage source.

I think you are most likely right on this.  The difference between the two circuits could simply be the characteristics of the individual electrolytic caps.  I noticed that the fastest settling time by far was with the film cap. 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on June 08, 2018, 05:22:28 pm
REPORT ON FINAL 10/100K:1 AMPLIFIER PERFORMANCE.

I got the revised PCB in recently from JLCPCB.  I built up the board using new components except for the Electrolytic capacitor and the largest film caps (2.2uF and 22uF) which I salvaged from one of my prototypes.  The board worked with no issues.   Performance is slightly better than my best previous prototype.  The difference is probably due to superior board layout, especially since I re-used the same Electrolytic capacitor, so capacitor leakage should be the same as the prototype. 

Noise floor tests.   Tests were done using a 50 ohm resistor across the input and also with a NiMH battery based low-noise source. 


To measure the noise of the amplifier itself, the short circuit case is more relevant than the open case.
The relevant case is with a low noise around 10V source (e.g. 8*NiMH AA cells at constant temperature).
Otherwise you do not get the real life noise due to leakage current of the input capacitor.
A short or open will give too low values for the noise floor.

Andreas

I bought 8x NiMH AA (EverReady) cells from Digikey and charged them with a new T4S Tenergy charger bought for this purpose.  I found the output voltage surprising noisy as shown in an attached scope waveform of the output on my 10K:1 amp.  I added some filtering components to clean up the output.  The battery was housed in a cast AL enclosure with BNC connector output.  Schematic for battery box is also attached.   

Noise floor of the new amplifier with 50 ohms across the input (10K:1 mode with 1K input resistance) was typically <120nV pk-pk. 
Nose floor of new amplifier using the low-noise NiMH source box was typically <150nV pk-pk.

I also measured a couple of LTZ-1000 based 10V references I have built recently.  One is housed in a battery backed Vref chassis and the other is just a PCB in a plastic bag on the bench.   Value of <1.5 uV were measured with the Vref chassis being a bit quieter as expected.

Some photos and board  As-built schematic are attached below.  This project has been a great learning experience and just another adventure traveling the rabbit hole of volt-nut pursuits. 

Insatman
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on June 09, 2018, 04:07:10 am

I bought 8x NiMH AA (EverReady) cells from Digikey and charged them with a new T4S Tenergy charger bought for this purpose.  I found the output voltage surprising noisy as shown in an attached scope waveform of the output on my 10K:1 amp. 

I usually put the battery and the amplifier into a (grounded) cookies box. (only the BNC-line to the scope runs outside).

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on June 09, 2018, 06:48:00 am
Newly charged batteries might show a little more noise and drift. It can take some time for the chemistry to settle and this might not always be smooth.  Similar it might take quite some time for the input cap to settle with the new voltage.

It is very unusual to need filtering after the battery.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on June 09, 2018, 10:47:48 am
Newly charged batteries might show a little more noise and drift. It can take some time for the chemistry to settle and this might not always be smooth.  Similar it might take quite some time for the input cap to settle with the new voltage.

It is very unusual to need filtering after the battery.

It was approximately 16 hours after charge that the measurement labeled RAW BATT was made.   I was thinking along those lines, but decided that a filter, so long as it used FILM caps, would probably be helpful.  I know it takes the output voltage a bit of time to settle  now, but that is dwarfed by the time the electrolytic cap takes to settle in the amplifier anyway.  Perhaps these EverReady cells are more noisy than most?   I don't have any experience with NiMH batteries in this realm. 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Gerhard_dk4xp on June 09, 2018, 11:12:21 am
These are two 18650 Li Batteries in series, abt. 7.5Vdc:
<    https://www.flickr.com/photos/137684711@N07/39056813010/in/album-72157662535945536/ (https://www.flickr.com/photos/137684711@N07/39056813010/in/album-72157662535945536/)    >

0 dB is 1nV/rt Hz, the batteries hit my measurement limit. I intend to move this limit further
both by using a chopper amplifier and by using cross correlation.
There are other plots in this album that cover barefoot & filtered LT6655 and LEDs.

Other batteries:
<   http://www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf (http://www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf)     >

The worse than 1/f rise of the noise below 50 Hz goes on the pre amp that had too small a coupling
capacitor at the time the measurements were made. That has been healed with a big wet slug tantalum
in the mean time. The FFT analyzer also adds low frequency noise.

There are more papers on this subject in that directory.

The NIST paper given in the reference is interesting also, at least as long as it does deal with
voltage noise only.

12V 12AH Pb will follow this weekend when the weather is bad here, i.e. I don't need my motorbike
for driving around and can lend it's battery.

regards,
Gerhard
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on June 09, 2018, 04:06:42 pm
It is very unusual to need filtering after the battery.

Thats true.

I also think that the input capacitor was not charged long enough. (2-3 days)
I always keep the input capacitor charged to about 10V except for noise measurements.

With best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on June 10, 2018, 11:46:15 am
It is very unusual to need filtering after the battery.

Thats true.

I also think that the input capacitor was not charged long enough. (2-3 days)
I always keep the input capacitor charged to about 10V except for noise measurements.

With best regards

Andreas

I will leave the unit's input connected to the NiMH battery (x8 in series w/ filter) for 3 days and repeat the noise measurements.

Insatman
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Insatman on June 14, 2018, 05:55:35 pm
New 10000:1 amplifier noise measurements using NiMH battery as source.
After 3+ days biased at 10V I got slightly lower noise measurements than the previous measurements.   Prior measurements were typically <150nV, now after 3+ days of being biased at 10V, the typical number is <130nV.  This is approaching the noise floor when the amplifier is shorted by a 50ohm terminator.  Still the difference of 20nV, while significant, isn't going to affect measurements of a =>1uV very much.  Conclusion is that keeping the amplifier biased at 10V is a good idea, but not essential for making good measurements so long as the noise level you are measuring is >1uV.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: vindoline on June 21, 2018, 11:16:49 am
I'm thinking that a low freq LNA is a god next project. Is there any consensus amoung the group which one to build? I'm not up to designing my own from scratch (yet!), but I'll probably lay out my own board to fit my case of choice. My use will be to measure my D.C. reference noise and compare it to the US cal club ref when it comes along my way.
Thanks in advance.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: flittle on October 16, 2018, 04:14:43 pm
Does anyone have any extra LNA PCBs?  I would be happy to purchase them with shipping costs of course.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Echo88 on October 16, 2018, 06:28:32 pm
User mimmus78 might still have a few. I got two PCBs from him which work very well.  :-+
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on October 16, 2018, 07:51:20 pm
Thanks Echo88, yes I still have few ones ... and some time to ship now.

Anyway Great part of the merit should go to Andreas that published his schematic ...
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: flittle on October 17, 2018, 07:26:20 am
PM sent mimmus78. 
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: mimmus78 on October 19, 2018, 10:35:45 pm
Hi guys, all pcbs based on Andreas design are gone right now and travelling across Europe.
Next time I will do some others pcb for me I will respin the boards ...
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: RandallMcRee on November 10, 2018, 12:57:04 pm
Recently made some measurements of my equipment using a Pipelie LNA (s/n 1157A). Values below are RTTI. Should be comparable to Zlymex results column 6.

Fluke 731B #1 avg = 2.5 uV p-p
Fluke 731B #2 avg = 1.7 uV p-p

Malone DMMCheck 5V output avg = 26.8 uV p-p

Ensemble of 7 LTZ1000 7.115V output = .39 uV p-p
Ensemble 5V output (custom resistive divider) = .35 uV p-p
Ensemble 10V output (opamp X2) = .87 uV p-p

My measurement setup is pretty basic. In particular, I have not shielded everything very well and I notice that readings increase when I am nearby in the room. Still, the Fluke 731B measurements suggest that I am not wildly wrong.

However it seems that my circuits do not preserve the low noise of the ensemble very well. Straightforward ratios would suggest
7V output = 1.2uV/sqrt(7) = .45 uV <ideal>    <--did ok here!
5V output = 0.39/1.43 = 0.27uV <ideal>        <-- marginal
10V output = 5V*2 = 0.55 uV    <ideal>        <-- very marginal, but good compared to zlymex table right?

C27 and C28 were bodged in after measuring worse noise figures. C27 helped about 10% and C28 another 10%. Also deleted an unused LTC1043 (left over from an experiment to get 10V using it, but replaced as shown) and gained another 5% decrease.

Any thoughts on taking this further or should I just count my blessings? The above is a result of several months of work.

Should I try deriving the 10V output directly from the 7V output?  (I won't do this lightly since preserving the tempco of the ensemble will require something like a VHP100 voltage divider, $$$)

Has anyone tried something like this? Experience to share?

Also:
LNA noise floor = .23 uV p-p measured in this setup (specs say .1uV so perhaps there is a problem here?)

Thanks!
Randy
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on November 10, 2018, 06:57:19 pm
Hello,

for the results:
how often did you repeat each measurement?
how was the standard deviation of the measurement?
how long was each measurement (10 s or 100 s)?

For 10s measurements I typically have a stray or +/-20 .. 30%.
And this under ideal conditions (DUT + amplifier in cookies box).

I usually do 15-20 repeated measurements and calculate average + standard deviation.

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Echo88 on November 10, 2018, 08:15:10 pm
Very interesting results Randall :-+

Could you please elaborate on your 7xLTZ-Ensemble and the 5V/10V-ensemble? Did you connect the 7 LTZ-references via the 2k-input-resistors to get the 5V/10V?
Nobody answered yet how much the soakage time of the LNA-input-caps matters/how big the noise-difference is; did you wait long enough after connecting the 7.1/5V/10V-ensemble to the LNA?
I cant say anything about the circuit and can only point to the AN159, which describes how to build a nice shielding enclosure.
I also have a 0.1-10Hz-LNA, but need to build a testbox with various voltage-references as a sanity-check.

A more detailed review of your measurements with pictures would be nice, if you have time.  :-DMM
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on November 10, 2018, 08:22:05 pm
The peak to peak measurements always show quite some scattering. It is noise one is measuring after all, so it is not predictable.
To get accurate values it takes averaging - even with 25 intervals of 10 s averaged the scattering is expected to go down by a factor of 5 only. With little time in between chances are averaging might be even a little less effective.

Looking at the RMS value is a little faster. Here the scattering is smaller to start with. However the factor between RMS and peak to peak values is not absolutely constant (due to correlation effects of things like popcorn noise). Still the RMS values gives a faster and less scattering indication of the noise.

The circuit looks reasonable. One point that might contribute to extra noise could be an interaction between the chopper stabilized OPs.

The LNA noise floor looks like a little on the high side. Is this measured open circuit or short circuit ? The open circuit noise is expected to be higher and does not apply to the later use.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: MiDi on November 10, 2018, 09:13:45 pm
The LNA noise floor looks like a little on the high side. Is this measured open circuit or short circuit ? The open circuit noise is expected to be higher and does not apply to the later use.

The scope adds too, my DS1054Z @ 20MHz BW & HiRes adds ~35nV (http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1428673/#msg1428673) (referred to x10k LNA input) - without BW limit and normal aq mode this value is higher.

Without proper shielding of LNA I got ~100nV extra noise (http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1423418/#msg1423418) with weird artefacts.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on November 10, 2018, 09:37:27 pm
Hello Midi,

is it possible on the 1054z to add a 1kHz software filter additionally to the 20 MHz BW limiter?
On my scope I can either select HiRes (generating 4 bits more resolution) or a programmable low pass filter.

With best regards

Andreas

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: MiDi on November 10, 2018, 10:25:49 pm
AFAIK nope, only option is HiRes and 20MHz BW-limit.
I think the 20MHz are software limit, like the 50MHz software limit with option for 100MHz.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: RandallMcRee on November 11, 2018, 05:22:47 am
Hello,

for the results:
how often did you repeat each measurement?
how was the standard deviation of the measurement?
how long was each measurement (10 s or 100 s)?

For 10s measurements I typically have a stray or +/-20 .. 30%.
And this under ideal conditions (DUT + amplifier in cookies box).

I usually do 15-20 repeated measurements and calculate average + standard deviation.

with best regards

Andreas

I should have said something in the original post--I took Vp-p readings from Rigol 1054Z set to 1s/div for a 12 second measurement, total. I then cherry-picked measurements  that did not have me walking about. So very unscientific, but when near the gear everything goes up by a factor of two to three. I took seven to nine Vp-p measurements by hand and averaged them.
The capacitor forming did not seem to be an issue (It does take quite a while to switch from one voltage to another). For example, this morning I am repeating the 10V measurement and the LNA has been connected since yesterday. The readings are perhaps slightly lower. The major uncertainty is me being in the room and air drafts, as well as the LNA noise floor, which as has been pointed out is definitely a factor of two too high.

A scope shot of this mornings readings of 10V output. DC value is 9.9999973 volts (maybe--not cal'ed). Second scope shot is "drafty". I don't see those three-five second excursions from across the room. If I take the cover off the enclosure--oh my, threw the roof! As expected I guess.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: RandallMcRee on November 11, 2018, 05:51:47 am
Very interesting results Randall :-+

Could you please elaborate on your 7xLTZ-Ensemble and the 5V/10V-ensemble? Did you connect the 7 LTZ-references via the 2k-input-resistors to get the 5V/10V?
Nobody answered yet how much the soakage time of the LNA-input-caps matters/how big the noise-difference is; did you wait long enough after connecting the 7.1/5V/10V-ensemble to the LNA?
. . .

Schematic for the ensemble part of the circuit attached. It ties in to what was previously posted. This shows the OPA140 low-pass filter I am using. The reason for the low-pass filter (optimized for low noise using Analog Filter Designer tool) is because I have this idea to compensate for LTZ anomalies e.g. long-term drift by detecting the difference between the LTZ input at the filter and the filter output. Pretty crazy. Not sure if this will ever work, but I'm including this in the interest of full disclosure because the OPA140 is not a low-tempco creature. The inputs to the filters are six PX (jason's PX LTZ1000 reference) and one KX (TiN's board). Two LTZ1000A and five non-A.

The Pipelie/Zlymex LNA has a "ready-to-go" signal. It appears to work. Yes, it takes quite some time.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: RandallMcRee on November 11, 2018, 05:57:02 am
The LNA noise floor looks like a little on the high side. Is this measured open circuit or short circuit ? The open circuit noise is expected to be higher and does not apply to the later use.

The scope adds too, my DS1054Z @ 20MHz BW & HiRes adds ~35nV (http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1428673/#msg1428673) (referred to x10k LNA input) - without BW limit and normal aq mode this value is higher.

Without proper shielding of LNA I got ~100nV extra noise (http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1423418/#msg1423418) with weird artefacts.

I measured closed circuit. As you both imply, seems like I need to put the LNA into the enclosure. Sigh.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: MiDi on November 11, 2018, 06:42:20 am
I measured closed circuit. As you both imply, seems like I need to put the LNA into the enclosure. Sigh.

Think this large unshielded loop area with clips from LNA input/BNC to DUT would introduce a lot of pick up noise.

LNA input to DUT should be as short as possible and fully shielded, best would be shielded twisted pair (I use cat 5e patch cable).

For your setup you could first try:
If possible put bare BNC direct onto jack of DUT, if not twist the clip and remaining wires as tight as possible together - should improve pick up noise and nearby sensitivity.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on November 11, 2018, 07:57:45 am
I measured closed circuit. As you both imply, seems like I need to put the LNA into the enclosure. Sigh.

Hello,

your signal on the scope above looks a bit fuzzy for a 10 Hz bandwidth limited signal.
The last time I had this there was a lot of mains hum on the signal by a transformer about 0.5 m away.
When I did a FFT of the noise, the 50Hz mains frequency was relatively dominant.

So it is always a good idea to make a noise floor measurement and a FFT of it before starting measurements.

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: branadic on November 12, 2018, 03:06:28 am
Can agree that noise can improve if reference is well shielded from ambient hum during measurement. Attached a comparison between reference at LNA on the lab table, but connections as short as possible (a) and reference with LNA inside a cookie box (b). In both cases there is no 50Hz hum visible. However, there is a clear difference.

-branadic-
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: RandallMcRee on November 12, 2018, 08:24:06 am
Folks,

Thanks for the suggestions, they have helped immensely.

I put the LNA scope cable into the enclosure and simply exited through hole in enclosure. This decreased the noise floor and air drafts ceased to be a problem. Simple and effective. I also found that the scope was triggering on the wrong channel, and this was causing fuzzy pics and slightly incorrect readings, so new pics and new measurements...

This time, to be more rigorous I took seven to nine sequential readings and averaged them, no cherry-picking  (No air drafts, as I mentioned).  Representative pics of each output attached, all on 2mV/div scale

Noise floor: 0.16uV p-p
7V output: 0.23 uV p-p
5V output: 0.34 uV p-p
10V output: 0.59 uV p-p

Thanks again for the help,
Randy
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: branadic on November 12, 2018, 08:41:45 am
Looks better now, but there is still some variation in noise visible. However, a raw estimation of the LTZ1000 noise: 1,2µVpp/sqrt(7) = 0,454µVpp proves that you are in the right ball park.
In general, activate 20MHz bandwidth limit on your scope, decrease sampling rate, no need for 1MSps, set the timebase to 10s per divison and watch noise for several 10 seconds. This way you can see if it's really noise or noise plus some additional thermal variation and/or hum.

-branadic-
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Andreas on November 12, 2018, 09:27:16 am

Noise floor: 0.16uV p-p
7V output: 0.23 uV p-p
5V output: 0.34 uV p-p
10V output: 0.59 uV p-p


Ups.

The noise floor of the LNA should be at least a factor 3-5 below the values that you want to measure to get a error below 10%.
Note that noise adds by square-law. (sqrt of squares)

with best regards

Andreas
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: RandallMcRee on November 13, 2018, 09:08:54 am
Branadic, here are one minute noise measurements for the 10 volt output...

If I read these correctly, these show that, yes, it is noise.

Randy
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: David Hess on November 13, 2018, 11:56:08 am
One of the tricks I used when I last designed and built a very low noise DC to low frequency amplifier was to measure the output noise with the input shorted and then with the input connected across a known resistance which produces a known noise.  This provided a sanity check on my noise measurement methods.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Gerhard_dk4xp on November 13, 2018, 03:02:05 pm
Yes. You don't even need to know the exact gain.
60 Ohms equals 1nV/rtHz absolute at room temperature.

Gerhard
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: splin on November 14, 2018, 01:56:18 pm
Trollocks; I thought this had posted a couple of days ago but just realised it didn't.  :palm: It's been a bit overtaken by events, but since I went to the trouble I may as well post it anyway.

Presumably all the later measurements are with the extra caps added which limit the bandwidth to around 1 to 2 Hz - which will significantly reduce the resistor, op-amp voltage and current noise (none of which should have much 1/f noise), but rather less of the 1/f noise from the LTZ1000 references. But I can't be bothered to try and calculate the impact.

Recently made some measurements of my equipment using a Pipelie LNA (s/n 1157A). Values below are RTTI. Should be comparable to Zlymex results column 6.

Fluke 731B #1 avg = 2.5 uV p-p
Fluke 731B #2 avg = 1.7 uV p-p

Malone DMMCheck 5V output avg = 26.8 uV p-p

Ensemble of 7 LTZ1000 7.115V output = .39 uV p-p
Ensemble 5V output (custom resistive divider) = .35 uV p-p
Ensemble 10V output (opamp X2) = .87 uV p-p

My measurement setup is pretty basic. In particular, I have not shielded everything very well and I notice that readings increase when I am nearby in the room. Still, the Fluke 731B measurements suggest that I am not wildly wrong.

However it seems that my circuits do not preserve the low noise of the ensemble very well. Straightforward ratios would suggest
7V output = 1.2uV/sqrt(7) = .45 uV <ideal>    <--did ok here!
5V output = 0.39/1.43 = 0.27uV <ideal>        <-- marginal
10V output = 5V*2 = 0.55 uV    <ideal>        <-- very marginal, but good compared to zlymex table right?


Assuming ensemble measurment is at output of U23A, noise over .1 to 10Hz:

1) 7.1 to 5V stage:
a) .39/1.43 = .27uV
b) U23B, Vn = 117nV pp x noise gain (1x)
c) 2112//5000 = 1484 ohms = 102nV pp thermal noise
d) U23B In- 16pA x 2k ohms = 32nV pp
e) U23B In- 16pA x 1484 ohms = 24nV pp

RSS noise = .32uV pp. Not too far from .35uV from your earlier test - it's not clear if your revised measurements are including C27 and C28, so I ignored those.

2) 5 to 10V stage:
a) 5V stage noise .35uV pp
b) U24 Vn 117 nV pp x gain (2x) = 234nV pp
c) 10k//10k = 5K ohms = 188nV pp x gain (2x) = 376nV pp
d) U24 In+ x 10 ohms - negligable
e) U24 In- 16pA x 5k x gain (2x) = 160nV p

RSS noise = .84uV; again not far from your .87uV.

Better, from a noise perspective, would be to generate the 10V from the 7.1V avoiding the added noise of the 7.1V to 5V stage. The drift of the 10V output relative to the 5V would likely be greater as the two stages can drift in opposite directions, but the 10V drift would be lower than the current scheme (given similar performance scaling resistors) as a) the ratio is lower and b) the 7.1 to 5V stage drift is eliminated.

Using lower value resistors would help - but DSMZs don't come cheap so filtering with added caps is a pragmatic solution. But when you are looking at low level, very low frequency noise signals, should you worry about dielectric absorption and temperature dependence of those caps adding their own 'noise'? (ie. not actually noise, but hard to distinguish changes from the low frequency noise and drift of the references).
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on November 14, 2018, 06:06:40 pm
For the reference circuit one does not have to worry about dielectric absorption (DA) very much, as the voltage the caps see does not change very much. The DA in film caps has a time constant in the 10-100 s range - so waiting that long is reasonably easy. It is quite a bit longer for electrolytic caps however. But still those high accuracy ref usually needs some warm up anyway.

The DA of an electrolytic cap at the LNA input however can be at least annoying, as it would take longer to settle when changing the voltage to test. If there are different long time constants involved (I don't know, but well possible) it can be even difficult to see when settling has finished.

The other point can be a temperature or mechanical effect on capacitors that can cause a voltage change, a little like thermal EMF fluctuations. I would expect this mainly with the electrolytic caps.

For the 5 to 10 V amplifier, there could be some extra noise from interaction between the AZ OPs.  Especially those ADA4522 in separate cases can have a clock the is close, but usually not in sync.  As it is a similar frequency residual spikes from one AZ OP can disturb other AZ OPs of similar type.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: RandallMcRee on November 16, 2018, 04:05:02 pm
So I built a second 7-to-10V circuit using Vishay dividers and an ADA4522-1. The input is directly from the LTZ1000 ensemble.

The noise is lower, around 0.3uVp-p.  Pics attached.

Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: 3roomlab on November 18, 2018, 07:28:14 am
has anyone noticed this blog? (fresh new JFET kids to make more LNA anyone?)
http://tech-blog.sblo.jp/article/183433346.html (http://tech-blog.sblo.jp/article/183433346.html)

(http://eh-field.sakura.ne.jp/sblo_files/tech-blog/image/All.png)

Mr tzukasa yasui's noise test on the new N Jfet (June 2018).
the 5908 are duals in a SOT26, it is going to be a very hot JFET, the singles version is apparently 2SK3557

*bonus*
http://tech-blog.sblo.jp/article/180081088.html (http://tech-blog.sblo.jp/article/180081088.html)
(http://eh-field.sakura.ne.jp/sblo_files/tech-blog/image/BJTNoise-thumbnail2.png)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: pelule on November 18, 2018, 07:40:27 am
Here the link to the english version: http://tech-blog-en.sblo.jp/article/183391570.html (http://tech-blog-en.sblo.jp/article/183391570.html)
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: chuckb on November 23, 2018, 08:48:34 am
It appears some choppers do not completely eliminate the basic flicker noise. The below link shows the limits of my favorite op amp, the ADA4522. I will need to find a new favorite. The ADA4528 looks good but it's a 5V part. This results are similar to what I experienced when using 16 ADA4522s in parallel for a 60dB DSA preamp. I had a nice 1.5nV / rt Hz white noise but I still had flicker noise below 1Hz.
https://ez.analog.com/amplifiers/operational-amplifiers/f/q-a/102479/ada4522-1-f-noise

This other paper by David Hoyland (2016) documents the flicker noise performance of several other chopper opamps below 0.1Hz. It also has the current noise spectrums.
https://dcc.ligo.org/public/0126/T1600206/001/Opamp%20Noise%20Test%20Results.pdf

If you need performance below 0.1Hz don't assume just any chopper will remove the internal flicker noise.

So far -
flicker noise free
ADA4528 (5v)

Low Frequency Flicker noise corner, less than 0.1Hz
CS3002
ADA4522
OPA188
OPA180

Has anyone tested other choppers below 0.1Hz?
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: branadic on November 23, 2018, 09:03:44 am
Thanks for sharing. I wonder how LTC2057 competes in this respect.

EDIT: Maybe similar to the latest LTC2058, with the input voltage noise spectrum given from 0.1Hz - 10MHz on page 7, which by the looks of it appears to be flat.

-branadic-
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: David Hess on November 23, 2018, 03:29:18 pm
The ADA4528 link identifies the likely problem, noise from thermocouple junctions.  Besides using dummy parts to equalize the number of junctions and their potential, drafts need to be eliminated to maintain an isothermal environment.

Even the precision of amplifiers like the OP-07 can be compromised by drafts so at least air baffles should be used where this is important.

Care should also be used to not load the amplifier output because this will limit open loop gain and cause drift from die heating; if necessary, an external transistor or integrated buffer is very useful for this.  This should be less of a problem with chopper stabilized amplifiers but I would do it anyway.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: chuckb on November 23, 2018, 04:29:08 pm
I have reached out to TPeter through AD Engineer Zone and invited him here to discuss the results.

My impression was that the test board had an ADA4522 and also an ADA4528 on it that were powered from the same power supplies. The amplifier outputs went to two channels of a spectrum analyzer. A thermal EMF issue should be an equal opportunity noise source.

I will set up a test myself in a few week when I'm back from vacation. I have also started to do some Chopper input current spike evaluations with a 100MHz BW transimpedance monitor. I definitely can see the current spikes in detail. More later.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Kleinstein on November 23, 2018, 08:49:58 pm
Thermal EMF combined with temperature fluctuations is one source of 1/f like noise. Here the often higher power dissipation in the higher supply OPs can be a factor.
I don't think that dummy junctions will help much, as it would not be clear they see the same temperature. It is more about using a good thermal layout.

For the current noise, one can expect some flicker part, as there is some internal charge injection, that does depend on the supply voltage or internally regulate supplies. So there is some kind of supply voltage to bias/offset current coupling.
For the current noise, also details of the circuit could have an effect, like parasitic capacitance at the inputs. Another point is that the current noise at the 2 inputs is likely partially correlated. So it depends where and how to measure.

The flicker noise shown in the links so far still looks relatively low. So in most application it is still not such a concern.

With several AZ OP in parallel I would be slightly concerned of possible interaction / inter-modulation effects.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Echo88 on November 24, 2018, 05:03:30 am
Maybe this doc is relevant in this case: http://www.sensorsportal.com/HTML/DIGEST/january_2011/P_745.pdf (http://www.sensorsportal.com/HTML/DIGEST/january_2011/P_745.pdf)  :-//
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: antintedo on December 11, 2018, 06:54:40 pm
Here's my take on the project. Grounded input (scope noise floor 0.8mV):
https://i.imgur.com/wPVcXfr.png

I chose to use 400u MKP cap as the input capacitor. Settling time is excellent, 10V step takes only around a minute to stabilize. Overall pretty satisfying results, but the voltnut within demands more performance :-DD

There seems to be a problem with ADA4528 input current noise or my design of the input stage. Initially two channels were paralleled, but the resulting noise floor was noticeably larger than using just a single channel (image (https://i.imgur.com/BHy7AcB.png)). I wonder if I got ICs from an underperforming batch or SMD metal film resistors I chose affect the performance so much.
Title: Re: DIY low frequency noise meter and some measurement result of voltage references
Post by: Echo88 on December 11, 2018, 09:17:01 pm
Like described in the thread you may characterise your amplifier:
sweep across the frequency range of the LNA with a defined constant input amplitude to see the real amplification of the amp instead of assuming it,
make a high resolution FFT to see if 50Hz couples to strong in the LNA and that way artificially increases your noise when viewed with the oscilloscope,
test a few different noise sources to do a sanity-check: different voltage references like a LM399/REF102/ADR445/LTC6655, batteries, short

Only that way you will truly know your LNA and can actually reliably work with it. Otherwise you will doubt the results and it may have problems you dont know about.

Only a guess: did you use a dual channel ADA4528-2 or single channel ADA4528-1 and maybe now see intermodulation-related errors?  :-//