Author Topic: DIY low frequency noise meter and some measurement result of voltage references  (Read 18774 times)

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Offline zlymex

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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

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.


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:
« Last Edit: May 21, 2016, 01:29:12 PM by zlymex »
 
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Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #1 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.


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.


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.


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.


Even if everything is idling, there is still exit low frequency wobbling probably because the very large TempCo or stress release or something.


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.


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.


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:


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

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.
« Last Edit: June 14, 2016, 02:04:56 AM by zlymex »
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #2 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.


When current increased to 11.8mA, noise is reduce even further to 236nVpp.

4. Other measurement result is summarize in table below



5. Ordered by noise


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?


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.
« Last Edit: May 21, 2016, 02:44:44 PM by zlymex »
 

Offline Andreas

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Re: DIY low frenquency noise meter
« Reply #3 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
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #4 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.
 

Offline Kleinstein

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Re: DIY low frenquency noise meter
« Reply #5 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.
 
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Online Vgkid

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Re: DIY low frenquency noise meter
« Reply #6 on: May 12, 2016, 03:07:16 AM »
Thanks for the circuit. Do you have any information on those 2dw233 zener diodes?
If you own any North Hills Electronics gear, message me.
 

Offline SeanB

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Re: DIY low frenquency noise meter
« Reply #7 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.
 

Offline Marco

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Re: DIY low frenquency noise meter
« Reply #8 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. Seems a simple planar zener, the third pin is superfluous.
 

Offline Andreas

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Re: DIY low frenquency noise meter
« Reply #9 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
« Last Edit: May 12, 2016, 07:37:02 AM by Andreas »
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #10 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.
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #11 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
Noise comparison tests
 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=119921
http://bbs.38hot.net/forum.php?mod=viewthread&tid=120264
Teardown and analysis 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/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.
 
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Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #12 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.
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #13 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. 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
« Last Edit: May 21, 2016, 02:54:51 PM by zlymex »
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #14 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.
 

Offline Andreas

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Re: DIY low frenquency noise meter
« Reply #15 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
« Last Edit: May 12, 2016, 03:35:16 PM by Andreas »
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #16 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.
« Last Edit: May 13, 2016, 12:58:29 AM by zlymex »
 

Offline Andreas

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Re: DIY low frenquency noise meter
« Reply #17 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
« Last Edit: May 12, 2016, 08:15:18 PM by Andreas »
 

Online Alex Nikitin

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Re: DIY low frenquency noise meter
« Reply #18 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
« Last Edit: May 12, 2016, 09:41:04 PM by Alex Nikitin »
 

Offline DuPe

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Re: DIY low frenquency noise meter
« Reply #19 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
« Last Edit: May 12, 2016, 10:10:44 PM by DuPe »
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #20 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.
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #21 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).
« Last Edit: May 15, 2016, 01:21:20 AM by zlymex »
 

Offline Andreas

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Re: DIY low frenquency noise meter
« Reply #22 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




« Last Edit: May 13, 2016, 02:12:55 AM by Andreas »
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #23 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.
 

Online Alex Nikitin

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Re: DIY low frenquency noise meter
« Reply #24 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
 

Offline Andreas

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Re: DIY low frenquency noise meter
« Reply #25 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
 

Offline Kleinstein

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Re: DIY low frenquency noise meter
« Reply #26 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.
 

Offline Andreas

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Re: DIY low frenquency noise meter
« Reply #27 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
« Last Edit: May 13, 2016, 07:19:48 AM by Andreas »
 

Offline DiligentMinds.com

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Re: DIY low frenquency noise meter
« Reply #28 on: May 13, 2016, 07:55:12 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.

BTW--- the LTC2057 suppresses 1/f noise.  It's LF noise spec is from DC-10Hz because of this.
 
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Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #29 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?
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #30 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.
 

Offline DiligentMinds.com

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Re: DIY low frenquency noise meter
« Reply #31 on: May 13, 2016, 11:13:14 AM »
Did anyone notice that the 9V battery is backwards on the LTC6655 in AN124?  Jim W. is pulling pranks even from the grave...
 
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Online Alex Nikitin

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Re: DIY low frenquency noise meter
« Reply #32 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
 
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Offline Andreas

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Re: DIY low frenquency noise meter
« Reply #33 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/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
« Last Edit: May 13, 2016, 08:46:40 PM by Andreas »
 
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Offline alanambrose

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Re: DIY low frenquency noise meter
« Reply #34 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
“A foolish consistency is the hobgoblin of little minds"
 
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Offline DiligentMinds.com

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Re: DIY low frenquency noise meter
« Reply #35 on: May 14, 2016, 12:40:58 AM »
Nice paper on capacitor leakage...

-Ken
 
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Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #36 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.
« Last Edit: May 15, 2016, 03:05:09 AM by zlymex »
 

Offline DuPe

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Re: DIY low frenquency noise meter
« Reply #37 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
« Last Edit: May 14, 2016, 06:02:28 PM by DuPe »
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #38 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.
« Last Edit: May 14, 2016, 09:04:56 PM by zlymex »
 

Offline DuPe

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Re: DIY low frenquency noise meter
« Reply #39 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
 

Offline splin

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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.
 

Offline zlymex

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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.
 
 

Offline Kleinstein

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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.
 

Offline zlymex

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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
« Last Edit: May 17, 2016, 09:05:35 AM by zlymex »
 

Offline alanambrose

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“A foolish consistency is the hobgoblin of little minds"
 

Offline zlymex

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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

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
« Last Edit: May 31, 2016, 11:48:02 AM by zlymex »
 

Offline alanambrose

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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

« Last Edit: June 01, 2016, 08:02:19 PM by alanambrose »
“A foolish consistency is the hobgoblin of little minds"
 

Offline zlymex

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.......
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.
 

Offline Kleinstein

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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.
 

Offline splin

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Re: DIY low frenquency noise meter
« Reply #49 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.
 

Offline David Hess

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Re: DIY low frenquency noise meter
« Reply #50 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.
 

Offline Andreas

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Re: DIY low frenquency noise meter
« Reply #51 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

 

Offline Kleinstein

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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.

 

Offline splin

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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.
 

Offline David Hess

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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.
 

Offline Andreas

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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
« Last Edit: July 20, 2016, 07:34:04 AM by Andreas »
 

Offline David Hess

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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.
 

Offline Kleinstein

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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.
 

Offline zlymex

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.......
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.
 

Offline zlymex

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Re: DIY low frenquency noise meter
« Reply #59 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.
 

Offline zlymex

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.....
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".
 

Offline David Hess

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...

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?
 

Offline zlymex

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...

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.
 

Offline Kleinstein

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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).
 

Offline zlymex

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The noise peak is at 3 kHz, not 3 Hz - so not really a problem with LF noise.
....
Oh yes.
 

Offline David Hess

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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?
 

Offline Edwin G. Pettis

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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!
 

Offline acbern

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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.
 

Offline zlymex

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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
 
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Online lukier

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Very nice compact design. Is your friend planning to sell some by any chance?
 

Offline zlymex

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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.
 

Online Alex Nikitin

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I would buy one as well.

Cheers

Alex
 

Online pelule

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I would buy one as well.

Cheers
PeLuLe
You will learn something new every single day
 

Online TiN

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And I would buy 2.
Does your friend plan to release gerbers? I could make boards :-]
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Offline DiligentMinds.com

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Hmmmm...  A new op-amp from Linear Tech might be useful for this task:

LT6081

-Ken
 

Offline Andreas

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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
 

Offline zlymex

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Hmmmm...  A new op-amp from Linear Tech might be useful for this task:

LT6081

-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).
 

Offline zlymex

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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).
 

Offline DiligentMinds.com

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Hmmmm...  A new op-amp from Linear Tech might be useful for this task:

LT6081

-Ken
Thanks for the info. This op-amp 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).

This op-amp just came out early this month.  It has much higher gain and very good offset (and offset drift) for this kind of amp.  With the current noise being what it is, you would want this to have a very low input impedance, and balance the impedance in the (+) and (-) inputs [it's not a chopper amp].  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.  This would make a very good first-stage for a 1/f noise meter.  (Even better with a discrete dual-JFET LSK389 input stage).
 

Offline splin

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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]
 

Offline DiligentMinds.com

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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]

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.
 

Offline splin

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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.
 

Offline DiligentMinds.com

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Still, an improvement over the older LT1028...
 

Offline Kleinstein

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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.
 

Offline DiligentMinds.com

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whatever...
 

Online TiN

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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.
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Offline zlymex

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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.
« Last Edit: August 18, 2016, 11:28:56 AM by zlymex »
 

Offline BU508A

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Re: DIY low frenquency noise meter
« Reply #87 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

Andreas


“Chaos is found in greatest abundance wherever order is being sought. It always defeats order, because it is better organized.”            - Terry Pratchett -
 
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Online TiN

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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. Second is discrete one, using ADA4522-4, copycat of zlymex's design.



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



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
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Offline Kleinstein

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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.
 

Offline zlymex

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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.
 

Online TiN

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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. :)
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Offline zlymex

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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.
« Last Edit: September 28, 2016, 11:30:47 PM by zlymex »
 

Offline Kleinstein

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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.
 

Offline DiligentMinds.com

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@TiN:

On your MK-III schematic, the collector of QC3 should go to +15V.  The LSK389 might be a better choice for the input dual-JFET.

I think Nichicon sells low-leakage aluminum electrolytics that might work very well as an input cap.

The LTC2057 has lower current noise than the ADA4522, it might (or might not) work better in this application because of the rather large input resistance for the chopper.  You would have to do the engineering (or build one) to see which is better.

It might be wise to add a switch that when the circuit is off, the input cap is charged by a battery.  Another switch could be used to increase the input resistance when first connecting to a source to prevent capacitative overload.

On the MK-II schematic, QB2 should either be a 2N3904 (the way you have it connected) or if you use the 2N3906 the base and collector need to be swapped.

------------------

Just a personal preference, I like the MK-II the best-- very clean, simple, and easy enough to replicate.  I wonder how many of these amps we could get away with paralleling?  (Is 4 the limit)?
« Last Edit: September 28, 2016, 11:06:34 AM by DiligentMinds.com »
 

Offline 3roomlab

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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.



i was curious and took some time to try and understand the amp, should R5 be nearer to 42k?

if the arrangement of the front end is change so that RP is less than 10ohms total, will that effectively reduce the total noise output? (ie: larger C1 6600uF? smaller R1 330R? R3 goes to 10k from 101k?)
« Last Edit: October 26, 2016, 09:53:06 AM by 3roomlab »
i'm literally losing my memory, so i have to ask all kinds of stupid questions even if it looks like trolling, to know i have the right answer.
 

Offline Andreas

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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
 

Online TiN

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Well, finally I got around and did a layout of the board for MK-I (40x100mm FR4 4L).



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

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 2, 4 hours (max limit on YT for HD, so first 1hour is trunicated :((

« Last Edit: October 31, 2016, 06:07:42 AM by TiN »
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Offline quarks

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Somehow I totally missed this, so now it is bookmarked.
 

Offline Nuno_pt

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The mumbling from Illya is the best. :-)

Looks like I've 3 more boards to build slowly.
« Last Edit: October 31, 2016, 05:09:24 AM by Nuno_pt »
Nuno
CT2IRY
 

Offline Kleinstein

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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.

 
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Online TiN

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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
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Offline VintageNut

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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?
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Offline Andreas

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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

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

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
« Last Edit: November 01, 2016, 11:49:38 PM by Andreas »
 

Offline VintageNut

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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

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

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.
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Offline VintageNut

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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.
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Offline VintageNut

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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.

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Offline Kleinstein

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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.
 
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Offline VintageNut

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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. 
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Offline Kleinstein

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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.
 

Offline dr.diesel

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@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.   :-+

Offline VintageNut

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@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
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Offline David Hess

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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.
 

Offline 3roomlab

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Well, finally I got around and did a layout of the board for MK-I (40x100mm FR4 4L).




i was returning to the repair of the K2015 which i left stranded many moons ago, and it reminded me of something about the sensitivity of the input pins. should the input at the SOIC have some form of guard?
i'm literally losing my memory, so i have to ask all kinds of stupid questions even if it looks like trolling, to know i have the right answer.
 

Online TiN

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And here they come...



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



Bottom side.



Stackup cutout ;)

Now need to get some caps, opamps and get the thing going!  :popcorn: :bullshit:
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Offline gamalot

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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.  :)
 

Online TiN

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Got bit of time to assemble test module, using just one AD8248ARZ.

I think it's not working :)







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.





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.
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Offline bktemp

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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.
 

Online TiN

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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.
« Last Edit: December 06, 2016, 03:42:09 PM by TiN »
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Offline Kleinstein

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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.
 

Online TiN

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Without buffer, AC coupled output cannot drive the 50 ohm load.

Here's actual schematics in sim:



LTSpiceIV circuit file.

AD8428 model using spice from AD site.

Symbol for LTSpice IV
SPICE file
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Offline Kleinstein

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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.
« Last Edit: December 07, 2016, 04:51:00 AM by Kleinstein »
 

Online TiN

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Diode? You meant C5/C6?

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



Not everyday you have chance to see 707$ USD/1pcs capacitor, so here are the photos. Regular radial can caps and SMA connector are nearby for size comparison purpose.



After 10 more hours leakage current stabilized around 3.2 nA at 10VDC from K2400.
« Last Edit: December 07, 2016, 05:02:56 AM by TiN »
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Offline Kleinstein

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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.
 

Offline VintageNut

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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.
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Online TiN

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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.

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Offline doktor pyta

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@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.
 
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Online TiN

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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.



Also built a test box from cast enclosure with triax ports, so I can make semi-permanent setup to measure components using Agilent 4142B.
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Offline zhtoor

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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.

 

Offline zhtoor

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Re: DIY low frenquency noise meter
« Reply #129 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.


When current increased to 11.8mA, noise is reduce even further to 236nVpp.

4. Other measurement result is summarize in table below



5. Ordered by noise


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?


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.

 

Offline Andreas

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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

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
 

Offline Kleinstein

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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.
 

Offline Andreas

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they also use a very low current through the "zener" ...
 

Offline zhtoor

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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

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.
 

Online TiN

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Incoming....





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.
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Online TiN

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RSVD for tests.

Keithley 2400 Vsource , +9.5Vset.



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Online lukier

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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).  :-+
 

Offline Kleinstein

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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.
 
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Offline mimmus78

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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.



-> Check my Store @ Ebay
-> OR my Blog @ 118volt [IT][EN]
 

Offline mimmus78

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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?
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Online lukier

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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.
 

Offline David Hess

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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.
 

Offline Andreas

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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

With best regards

Andreas

Edit: link added
« Last Edit: March 02, 2017, 07:33:28 AM by Andreas »
 

Offline mimmus78

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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?
« Last Edit: March 04, 2017, 10:18:06 AM by mimmus78 »
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Offline Andreas

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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

« Last Edit: March 04, 2017, 04:36:23 PM by Andreas »
 
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Offline mimmus78

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Yes and cookies can be used during assembling :-) too
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Offline Kleinstein

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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.
 

Offline mimmus78

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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?

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Offline Andreas

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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

With best regards

Andreas
« Last Edit: March 04, 2017, 10:07:19 PM by Andreas »
 

Offline mimmus78

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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:


Messy - Back:


1uV RMS - 15Hz


4AA alkaline battery noise

« Last Edit: March 12, 2017, 01:16:25 AM by mimmus78 »
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Online TiN

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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.
xDevs.com | Have test gear documentation to share? Upload here! No size limits, firmware dumps and teardown photos welcome.
 

Offline Andreas

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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
« Last Edit: March 12, 2017, 02:38:00 AM by Andreas »
 

Offline mimmus78

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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.



PS: what happens when I run out of batteries with this circuit?
« Last Edit: March 12, 2017, 07:22:31 AM by mimmus78 »
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Offline Andreas

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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?
« Last Edit: March 12, 2017, 05:49:03 PM by Andreas »
 

Offline mimmus78

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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.



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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« Last Edit: March 13, 2017, 09:14:55 PM by mimmus78 »
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Offline Andreas

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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
 

Offline mimmus78

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Andras it was 2.1mF measured at 100Hz.
Anyway you already had -3dB at 0.1Hz with 3mF in your simulation ... not much unexpected.
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Offline mimmus78

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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?




« Last Edit: March 16, 2017, 08:24:04 AM by mimmus78 »
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Offline Andreas

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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


 

Offline mimmus78

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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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Offline Kleinstein

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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.
 

Offline mimmus78

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@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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Offline Andreas

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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
 

Offline mimmus78

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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.
« Last Edit: March 19, 2017, 11:36:22 AM by mimmus78 »
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Offline Andreas

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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
 

Offline mimmus78

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Andreas it was the third PCB I routed and got fabricated. This was the main part I didn't trust. 😁

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Offline pmcouto

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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
 

Offline mimmus78

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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.
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Offline Kleinstein

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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.
 

Offline Andreas

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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

 

Offline pmcouto

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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
 

Offline pmcouto

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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
 

Offline mimmus78

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Nice!

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

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Offline Andreas

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Gerber files sent to PCB house.
PCB was designed to fit inside an aluminum Hammond enclosure (1590T).

nice design  :-+

with best regards

Andreas
 

Offline pmcouto

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Thanks  :)

I use Altium Designer.


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