Author Topic: Low frequency, very low level, DC biased, noise measurements  (Read 34302 times)

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Offline dom0

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #75 on: April 23, 2015, 05:09:55 pm »
Ah, now I get it! :)
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Offline paulie

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #76 on: April 23, 2015, 05:16:19 pm »
Yes, I agree putting it after is very confusing and makes things hard to follow. :)

There are situations where MOSFET does a great job and 2n7000 is one of my favorites. But in this case a JFET has significant advantages due to gate biasing difference. For example in a single supply follower the MOSFET fails miserably near ground but JFET soldiers on. That's assuming you account for the offset but at least it works.

So MOSFET generally higher impedance and JFET allows negative gate bias. For my ultra low leakage measurements (ion chamber) the 2n4117 was best of both worlds. BTW thanks Rupunzell for the info vs 2n3819, it explains a lot.

JFET's can do a lot of things, but beat a good CMOS at input current is not one of them.

PS. don't top post :)
« Last Edit: April 23, 2015, 06:36:00 pm by paulie »
 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #77 on: April 24, 2015, 04:35:08 am »
Thank you sir!  :-+

The circuit with the LT1037 used 3.2 mF and 1 K ground. Feedback for 100 times amplification via 10 K and 100 Ohms. Source protection is through a resistor at the input, that is manually bridged with a switch.

If higher frequency stability is a problem and no need for amplification above 1 kHz, a LT1007 (compensated for G=1) may be the better option. The resistor to ground could well be larger, as the 0.1 Hz lower is set by the 2nd stage. A larger resistor would not increase noise in the 0.1 Hz and up range, but only the settling time at the start.
Link to simplified circuit:
http://www.mikrocontroller.net/topic/207061#2060389
my2C
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Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #78 on: April 24, 2015, 05:31:20 am »
Recap:
  • Wanted to measure noise from the LTZ1000 after doing various changes. Noise around 250nVrms 0.1-10hz
  • Needed a low noise amp for that, with noise below 250nVptp or 100nVrms 0.1-10hz.
  • Need high gain, >= 100x to get decent signal output levels. So DC blocking needed.
  • Ruled out the option to bias amp as that introduces new noise.
  • Low input impedance needed on input to keep noise down and therefore very big input caps
  • So I started to measure leakage on big caps.
  • Needed an ultra low current amp (pA) to do that
  • And a temperature controlled box for everything
All these subjects have been covered, are interesting in any case so the discussions kept rolling.
  • I'm convinced I can reach the noise goals with op-amps, both bi-polars and jfet could be used
  • I'm building some hardware now, results coming after weekend.
  • Discreet front end could reach even better results, but would be too much of a learning curve for me. For now, not really needed.
The low noise and leakage current amps are still demanding. So suggestions need numbers, actual component values used, input impedance, type of op-amp/transistors, capacitor sizes, and results also for the 1/f domain. White noise on it's own is not really useful at 0.1-10hz.

There has been a lot of useful info in the thread! But no offence meant guys, for myself, my case for the LTZ1000 measurements, I feel like I'm beyond general good advise and opinions now, unless backed by fairly detailed descriptions, numbers, both for measurements and calculations.

Keep it rolling  :scared:
my2C
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Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #79 on: April 24, 2015, 05:41:27 am »
As far as I know, mosfet (opamps) have high 1/f low frequency voltage noise/corner frequency. Are there new good devises out there?

2N4117, excellent current leakage specs but the high voltage noise kept me away. Also the IDSS was confusing. The uA value is there but in graphs, results go several orders of magnitude higher? I have a big bag full of these since a surplus shop closedown...
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Offline Rupunzell

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #80 on: April 24, 2015, 06:37:47 am »
Yes, this was my impression. Some where along the way ultra low input bias op-amps got mixed up into the whole works and completely drove this "discussion" is beyond tangent ways.

Yes, CMOS input op-amps have low input bias current. They are not the answer to it all as the inputs can be zapped if abused. While the inputs are protected by a set of diode clamps, their leakage currents do not balance out at all power supply voltages resulting in an increase if input bias current and the capacitance at their inputs can result in frequency response and related dynamic problems. Their common mode rejection is not that great. Once the source impedance drops, their advantages disappear.

As for MOSFET type devices in general, the first large volume production of MOSFET based sensors were alpha particle based smoke detectors (circa 1980's) where the sensing was done using a P channel MOSFET like the 3N163/3N164. These devices are low inout current due the effectively insulated gate structure which resulted in input bias currents in the femto-amps over a broad range of temperature and conditions.
http://www.linearsystems.com/assets/media/file/datasheets/3N163-4.pdf

There is a LOT more to what is required to making the noise measurement device than just the input device or op-amp.


**Voltage references are not high impedance devices which negates the advantages offered by extremely low input bias current devices, discrete or integrated.***


Back to the initial question of measuring noise from a voltage reference that has relatively low impedance and lowish noise over a bandwidth of 10 Hz, Correct?

Two examples have been presented here as starting solutions. Linear Technologies AN124 and the other was using a LT1012 and LT1037 which appears to have good and reasonable test results. The way to proceed would be to build either the test fixtures as outlined in AN124 or the LT1012 ad LT1037 and see if they produce the results required. If the test results are not good enough, then proceed to improve upon what has been done as needed.

It's that simple.

If there are improvements that needs to be made to improve specifics such as noise floor, DC stability or etc, that is when specific questions can be asked with possible solutions suggested.

Set the intellectual/Mental masterbation aside, check the engineering ego at the door as none of that is helpful towards solving the initial question.


Bernice


Recap:
  • Wanted to measure noise from the LTZ1000 after doing various changes. Noise around 250nVrms 0.1-10hz
  • Needed a low noise amp for that, with noise below 250nVptp or 100nVrms 0.1-10hz.
  • Need high gain, >= 100x to get decent signal output levels. So DC blocking needed.
  • Ruled out the option to bias amp as that introduces new noise.
  • Low input impedance needed on input to keep noise down and therefore very big input caps
  • So I started to measure leakage on big caps.
  • Needed an ultra low current amp (pA) to do that
  • And a temperature controlled box for everything
All these subjects have been covered, are interesting in any case so the discussions kept rolling.
  • I'm convinced I can reach the noise goals with op-amps, both bi-polars and jfet could be used
  • I'm building some hardware now, results coming after weekend.
  • Discreet front end could reach even better results, but would be too much of a learning curve for me. For now, not really needed.
The low noise and leakage current amps are still demanding. So suggestions need numbers, actual component values used, input impedance, type of op-amp/transistors, capacitor sizes, and results also for the 1/f domain. White noise on it's own is not really useful at 0.1-10hz.

There has been a lot of useful info in the thread! But no offence meant guys, for myself, my case for the LTZ1000 measurements, I feel like I'm beyond general good advise and opinions now, unless backed by fairly detailed descriptions, numbers, both for measurements and calculations.

Keep it rolling  :scared:
« Last Edit: April 24, 2015, 06:49:24 am by Rupunzell »
 

Online Marco

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #81 on: April 24, 2015, 01:35:48 pm »
Hmm, but Janaf IS actually building stuff ... it's the rest of us mentally masturbating.

On that note, as I said before ... dielectric absorption is entirely irrelevant as long as it doesn't saturate the amplifier. Won't find capacitors with worse DA than EDLC, yet it's what they used to get pretty much to the JFET noise floor in actual measurements (I'm pretty sure they misdiagnosed the source of the extra noise with the normal high pass filter though, as far as I can see it's simply the 10K resistor Johnson noise no longer being attenuated by the capacitor).
 

Offline GK

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #82 on: April 24, 2015, 02:17:08 pm »
If you're measuring the noise of things like voltage regulators/references which have low output impedances (particularly at low frequencies), (in)sensitivity to current input noise is such that bipolar input op-amps are suitable here. bipolar op-amps generally have much lower 1/f corners than do jfet op-amps. It's trivial to make a sub 1nV/rootHz amplifier suitable for measuring LF PSU and voltage reference noise with multiple bipolar-input op-amp stages in parallel. I've done 0.4nV with a ~10Hz 1/f corner with multiple AD8599. The current/voltage input noise crossover was for a source impedance of approximately 100 ohms IIRC.

 
   
« Last Edit: April 24, 2015, 02:24:07 pm by GK »
 

Online Marco

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #83 on: April 24, 2015, 02:43:03 pm »
A recap for the people who obviously didn't read the previous 5 pages of mental masturbation :) The problem is the high pass filter, the source might be low impedance but the high pass filter isn't and thus we end up with the OPA140.

You can put the filter in the feedback path of the opamp, but then you pretty much need EDLC's ... which means you need to keep amplification low'ish (100x range) so the leakage current doesn't saturate the amplifier.
« Last Edit: April 24, 2015, 02:48:05 pm by Marco »
 

Offline TiN

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #84 on: April 24, 2015, 07:50:27 pm »
I can offer some help in PCB, as interested in this topic too, for exactly same reasons.
YouTube | Chat room | Live-cam | Have documentation to share? Upload here! No size limit, firmware dumps, photos.
 

Online Kleinstein

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #85 on: April 24, 2015, 07:56:59 pm »
Die impedance of AC coupling capacitor is a critical part. When using a BJT amplifier a large part of the low frequency noise can be from amplifier current noise times impedance of the capacitor. So a BJT based amplifier needs to use a large capacitor, likely an electrolytic one.

Leakage of the cap is not such a big problem. It may limit the amplification of the first stage, but not that much, as a large cap also allows for a lower resistor to ground. So even 1 µA of leakage at a 10 K resistor is only 10 mV at the input. Having a second stage may be needed for extra filtering anyway.  At least down to 0.1 Hz the E-cap. and BJT OP is still feasible, it just needs a large cap with reasonable low leakage and noise. It gets increasingly difficult as the lower frequency limit is lowered. Also lower voltage noise OP-Amps will also need even larger caps, as they have more current noise.

Putting the filter in the feedback path does not really help, except possibly isolating the source from the charge stored in the large cap. However input protection tends to have something like diodes over the OPs inputs and thus input current can still be a problem - so not much gained with this. Also the noise problem does not really change, its just at the other input.

The high quality JFET OP (like OPA140) is an option in spite of the higher 1/f noise limit.  The input cap. can be smaller (e.g. 10 µF) and thus a film type. Going lower in frequency only needs an linear increase in the cap size. Different form BJTs several OPs in parallel are a real option. It would need something like 3-4 OPA140 in parallel to get about the LF performance of an LT1037 with a 3 mF coupling cap.

So there are several (not just the 2 above) options that should work. Also having 2 ref. sources and a differential measurement is a way to get around the coupling capacitor at the input. For an extremely low noise source like a battery or chemical ref. cell there still is the option to go for two independent amplifiers and use cross correlation to compensate for the amplifiers noise.
 

Offline Andreas

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #86 on: April 24, 2015, 09:52:02 pm »
The input cap. can be smaller (e.g. 10 µF) and thus a film type.

Hello,

this would imply to have a input impedance of around 200K for the resistor.
You forget one fact: a 250K resistor already has a noise of 0.2uV RMS (or 1.2uVpp) for a 10 Hz bandwidth.
http://www.sengpielaudio.com/calculator-noise.htm

The noise that a volt nut tries to measure is the 1.2uVpp of a LTZ1000.
So the whole measurement system has to have at minimum a factor 3 less noise (or less noise than a 25K resistor alone).
So practical you cannot go above 10K with the input impedance.

With best regards

Andreas
 

Online Marco

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #87 on: April 24, 2015, 11:07:39 pm »
Putting the filter in the feedback path does not really help, except possibly isolating the source from the charge stored in the large cap.

It decouples resistor noise and time constant from input impedance ... when the HPF is at the input all three are interconnected and you are limited in your choices. It doesn't really matter here since say a 100kOhm/220uF combo has a reasonable time constant, below OPA140 noise at 0.1 Hz and presents a reasonable input impedance (22uF would work too, but with 10's of nV/rtHz of Johnson Noise from the effectively 50kOhm input resistance near 0.1 Hz you start adding a bit of noise to the OPA140 there).
 

Online Kleinstein

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #88 on: April 24, 2015, 11:43:48 pm »
True, having the AC coupling in the feedback gives a higher input impedance. However input impedance already is relatively high - its essentially set by the resistor to ground. As noted by EmmanuelFaure above, a larger resistor actually reduces the noise. So Its good to have a lower frequency limit a the input, and use a later stage (or software) to set the lower frequency limit. A very large resistor makes settling slower, so I would not go much above 100s for RC (without extra provisions), but also not below about 10s. So even with a large 10 mF cap at the input, something like 10 K input resistance is reasonable.

Anyway a LF noise measurement is nothing to do fast. Thermal settling likely also take several minutes or more.
 

Offline paulie

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #89 on: April 24, 2015, 11:58:57 pm »
Set the intellectual/Mental masterbation aside

Yeah... but it feels sooooooo goooooood! ngggnnga...ngannnmnn...
 

Offline Rupunzell

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #90 on: April 25, 2015, 05:15:44 am »
This paper:http://pessina.mib.infn.it/Biblio/Biblio_Articoli/IEEE%20TNS%20V%2047%20p%201851%201856%202000.pdf

Bolometer = thermo energy to power converter (in this paper, the Bolometer is modeled as a current source with an impedance in parallel with it).

Tries to account for coupling capacitor problems as noted in equation (2) with partial success. Looking at their model, they do not account for many of the real world non-linear realties of capacitors. As the value of the capacitor increases, the problem increase.

Parametric amplifiers use the reactance of capacitors (pumped varactor diodes. capacitors are essentially noiseless when near ideal but can accumulate noise) to achieve very low noise. Electrolytic capacitors are a very different kettle of fish.

Much of this paper re-states what has already been discussed.

*Single ended input section delivers lower noise than a diff pair, about 3db.

*When the source impedance is low, bipolar input devices are low noise. They are ideal when their noise current and noise voltage meet at a narrow range of source impedance. Note in Fig. 5, bipolar transistors are lower noise at low frequencies if optimized for source impedance.

*Once the source impedance increases, JFETs gain an advantage due to much lower to lack of noise current.

*Capacitors can be a source of error-distortion.

*The Moxtek cooled JFET (about 100 degrees K) combines very low noise with low input capacitance. This was one of the devices initially mentioned with a link to the data sheet.

*MAT-03 (PNP diff pair), which originally were made by Precision Monolithics now owned by Analog Devices are "super match" bipolar pairs have low noise with low source impedances specially at low frequencies. This was one in diff pair offering in the MAT series from PMI. Much the same applies to low noise bipolar op-amps and bipolar input devices.

This is a typical academia style text book design. They could have used a servo to control the DC drift of cooled input device and used another device to add gain which would result in a input section with much better performance while retaining the low noise performance of the Moxtek JFET. This is a rather interesting device as it can be had in a "nail head" or bare die package that is designed to be used in a sensor device. The substrate is also intended to be biased to aid gate conduction.

They could have used a cooled Germanium JFET that would have resulted in lower noise performance than the Moxtek JFET.

No details on actual physical construction of this amplifier which can make the difference between a good low noise amplifier or one that does not work at all.

Once the initial gain has been achieved and the problems of signal integrity has been lowered, filtering and other signal processing can be done with fewer problems. Combining the initial signal gain and conditioning often results in a variety of difficulties and problems. Or applying the output of the input section to the Tektronix 7A22, 5A22N AM502 or Stanford Research Systems SR560.

Last, given the noise levels of the voltage reference to be measured, good low noise op-amps are good enough and if the dynamic impedance of the voltage reference is below say 300 ohms, a low noise bipolar input device or op-amp would do quite well. The more difficult problem is likely DC drift over time and temperature.

This measurement is just not that difficult to do.


Bernice



 
« Last Edit: April 25, 2015, 05:59:06 am by Rupunzell »
 

Offline Rupunzell

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #91 on: April 25, 2015, 04:57:11 pm »
Another way to do this.

Use the voltage reference under test as part of the DC bias network for the input section of a discrete low noise amplifier complete with DC servo to compensate for DC drift over time. This way, there is no need for any coupling capacitor, can keep the feed back impedances low and can provide enough gain for further signal processing.


One more note-toid on parametric amplifiers from another time, they offer common mode range of +/- 200 volts with pico-amp input bias currents and very low noise. Mostly obsolete today, become a mostly forgotten technology yet quite interesting.



Bernice
 

Offline dom0

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #92 on: April 25, 2015, 06:01:44 pm »
Well then you have the big cap (t=RC) for the DC servo again, although it might not need to be as big with a good FET op amp. If you do the DC servo analog, of course. If it's digital not so much an issue.
« Last Edit: April 25, 2015, 06:06:51 pm by dom0 »
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Offline paulie

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #93 on: April 25, 2015, 08:48:27 pm »
In my endless search for low cost substitutes for expensive glass giga-ohm resistors I spent the morning upgrading my ultra high resistance measurement jig. It uses cap discharge time instead of actual voltage ratios and since this can takes many minutes or hours instead of display it logs the data for offline analysis.

Then it struck me as potential for cap leakage test and I gave it a try. Fixed R instead of cap. Sure enough perfect for the job. No problem calculating leakage for big or small caps to less than a percent. Basically just a FET feeding directly into the MCU analog pin. No need for the more complicated op amp circuit I posted earlier. Of course having an electrometer grade 2n4117 helped a lot compared to the old 2n3819 I was using before. Thanks again for the opportunity. Huge fun, this hi-z stuff.
 

Offline Rupunzell

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #94 on: April 26, 2015, 04:30:00 pm »
Those Victoreen Giga ohm glass resistors originally intended for HV work?

Jennings vacuum capacitors also comes to mind with those HV parts.
http://www.jenningstech.com/ps/jen/prodsearch.cgi?action=capacitors

That charge amplifiers should do pretty good at measuring capacitor leakage as cap leakage is essentially measuring change in charge.

If you're curious and want to tinker, find some old electrometer tubes and give those a go as these were what was used to measure charge before the days of FETs. Some of the first electronic ph meters used these tubes. Continue on this path leads to devices such as Geiger–Müller tubes, photo multiplier tubes and eventually image intensifiers including micro channel plates. This is all really great and interesting stuff. Oh, condenser microphones with polarizing voltages and electret condenser microphones fit into this topic too.


As for using the voltage reference as part of the bias network. The VR would float between two current sources (source & sink) of near identical current. One end of the VR would connect to the FET gate, the return end of the VR would end up at the virtual or floating summing junction of the common source, active loaded FET. This summing junction would also be where FB is applied. A second single device (bipolar) is direct coupled to the FET's drain with the second stage's BJT's collector being actively loaded. This active load current source will have it's current adjustable to allow for output DC level. This is where the DC servo's control (Either a simple single pole integrator or sample-hold control type) can be applied. Add a current gain section to promote voltage gain and apply overall FB. Then use a low noise diff amp-op-amp to further increase gain and produce an output. The overall scheme is not that different from Jim William's AN124 with the exception of a different input section using discrete devices instead of a composite op-amp.


One last item, some where in the early Siliconix data book was a very small die FET with an internal cascode that was designed to be used as a charge amplifier. Now that my curiosity is up, I'll need to look it up later then week. Siliconix did not make these or very long. These could be a better front end FET than the 2N4117 as they are specifically designed for this kind of work as he 2N4117.


Bernice
 

In my endless search for low cost substitutes for expensive glass giga-ohm resistors I spent the morning upgrading my ultra high resistance measurement jig. It uses cap discharge time instead of actual voltage ratios and since this can takes many minutes or hours instead of display it logs the data for offline analysis.

Then it struck me as potential for cap leakage test and I gave it a try. Fixed R instead of cap. Sure enough perfect for the job. No problem calculating leakage for big or small caps to less than a percent. Basically just a FET feeding directly into the MCU analog pin. No need for the more complicated op amp circuit I posted earlier. Of course having an electrometer grade 2n4117 helped a lot compared to the old 2n3819 I was using before. Thanks again for the opportunity. Huge fun, this hi-z stuff.
« Last Edit: April 26, 2015, 04:35:00 pm by Rupunzell »
 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #95 on: May 12, 2015, 03:34:23 pm »
Hi guys, been away for a couple of weeks.

As I wrote before, I did some measurements on capacitors / leakages, intended for DC biased measurements.

Now I made some more measurements, this time I charged the caps to 7.1V and let them stand for one week for self discharge.

The results are ballpark the same as before but "better" tanks to longer time and better procedure.

Still a polystyrene top the list followed by polypros. Still a RIFA (Kemet) PEH169 industrial elyth is giving really good results.

I attach results. The second last column is the cap type leakage characteristic, the last column scaled by capacitance.

Some previously tested caps have been excluded now. Before the tests, the caps had been charged and left to rest for two days. Still, several e-caps have had higher voltage now than a week ago, i.e. very strong memory effect / dielectric absorption. Those results are not included here. I will measure them again in a week but if the DA is that high, they will act as non-linear LP filters with very un-predictable characteristics.   

I also added a few new caps to the test set but it seems too early to draw conclusions:
  • Russian "wet slug" tantalum-silver, 25V, 300uF
  • More RIFA PEH169, 2200uF 63V
  • Motor starter polypro, 800uF
Preliminarily,
- one tantalum-silver cap was quite good, the other quite bad.
- the RIFA, one cap was quite good, the other quite bad.
- the motor starter polypro; really bad.
I will charge them up to high voltage / take them down and se if any of them recover.

A comment; one needs very high impedance meters for this. My DMM has >20Gohm input and this still discharges "small" caps, like 10uF in the order of 1mV / second which makes measurements a bit demanding, when the capacitor self discharge is only tens of millivolts in a week. Thinking of a terra-ohm input buffer and DMM synced FET switch.
my2C
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Offline dom0

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #96 on: May 12, 2015, 04:30:34 pm »
6 pV/s is a spectacularly low droop rate in my opinion. Very nice results!
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Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #97 on: June 01, 2015, 04:23:25 pm »
Hi again, not been very active here for a while. Now, I have a dozen caps that have been resting, charged, for a few weeks  :)

For the cap leakage measurements, LMC6001 sure looks like a good candidate as well as OPA129 and LTC2054HV. Will build & test soon...

For noise, the new AD8428 looks really good. Instrument amp with 40nV ptp 0.1-10Hz. Excellent. Downside as with all low noise ams is input current, typ in the order of 200nA but I have an idea how to solve that.
 
It has a fixed G=2000 so I'd have to charge a big cap to within 1mV of an LTZ1000 and measure the LTZ1000 without DC blocking.
my2C
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Online Kleinstein

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #98 on: June 01, 2015, 08:21:29 pm »
Having the extra cap is very similar to having the filter in the feedback loop. This moves the problem to the large cap and the current noise of the other input causes trouble. The 200 nA Bias current is also combined with significant current noise  - so the capacitor would need to be very large (e.g. F range), a achieve a low impedance even at 0.1 Hz. The big question is than whether the voltage of the capacitor is that stable. Buffering the capacitor only moved the problem to the buffer.

The more realistic approach would be having a view low noise batteries (instead of the capacitor) to compensate most of the voltage. so hat less than 1 V DC is left.
Then a moderate (e.g. 10-20 fold) DC coupled amplification is possible, and AC coupling after that.
Batteries are known to have a rather good noise level. If in doubt, testing is possible by switching view batteries in series with different polarities. Large capacitors (likely super-caps) are not that well understood and can be more dependent on history.

Measuring leakage of the caps is only a relatively easy indication in searching for low noise capacitors. Some leakage, comparable to the bias of the OP and even a little more is likely acceptable. I just would not spend much time in testing caps with leakage in the µA range, if you can get better ones.

Anyway the Noise from an LTZ1000 is not that low. It gets tricky only if it comes to measuring the noise of individual batteries or chemical reference cells.
 

Online Marco

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #99 on: June 01, 2015, 09:21:25 pm »
As I said before, even a 22uF/100kOhm HPF is hardly going to budge the noise on a typical OPA140.
 


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