DIY nanovoltmeters and ultra low frequency noise preamplifiers, who did it?

[branadic]

Quite an interesting topic that is currently being discussed on xdevs chat. Nanovoltmeters are great to intercompare multiple references with the 3-cornered had method. But they are expensive, a used 2182A is almost the price for a new one.
There are some very nice inspirations out there, even for low noise preamplifiers.

Senis NVM-01 NanoVolt Amplifier

Ultra-low Noise Chopper Amplifier with Low Input Charge Injection

Ultralow-noise chopper amplifier with low input charge injection

Ultra-low noise chopper amplifier with low input charge injection

Looking into seismic exploration even more interesting solutions can be found, one of the most detailed one is:

Ultralow Noise Low Offset Chopper Amplifier for Induction Coil Sensor to Detect Geomagnetic Field of 1 mHz to 1 kHz

Who has interesting links to papers and articles? Someone DIY'ed something similar already, probably with a fixed input range?

-branadic-

[David Hess]

That is pretty standard stuff for low impedance measurements.  Chopping is needed to control 1/f noise at low frequencies where the measurements need to be taken.  Jim Williams posted some examples of how to use a chopper stabilized amplifier to remove the 1/f noise of a precision amplifier to get the best performance of both.

Didn't they make super low charge injection chopper amplifiers using LDRs?  I haven't tried it yet but photo-FETs could be used to do the same thing.

[branadic]

That is pretty standard stuff for low impedance measurements.

Down to 3 mHz?

-branadic-

[Kleinstein]

There are Choppers based on LDRs, but due to the resistance of the LDRs the noise tends to be relatively high. So this is more like the 5-10 nV/sqrt(Hz) level and higher. There can still be some effective charge injection from cross conduction, as the switching is slow.

There are nV amplifier based on JFET switches and a transformer at the input - these can get really low in noise (well below 1 nV/sqrt(Hz), but charge injection needs a carefull trim to get it low. This could be a way for a DIY solution - it is old style, but relatively simple. AFAIK Keithley has such an amplifier for the K2001/2002. TiN can probably tell more on these.

There is some App. note from Maxim on a chopper amplifier build with CMOS switches at the input, they do bet pretty low noise, but not so shure about the charge injection.
An improved version could be found somewhere on this web page: http://www.janascard.cz/aj_Zakazkova_vyroba.html#USB

I have tried a similar (but using full wave chopper with 4 switches and thus lower noise - a bit like the input stage of the DA1281 DMM) circuit on the bread board, to get an idea if it works OK. The results were not that bad, though the bread board is obviously not the way to do nV stuff. So the main point was that is was stable and not oscillating and the charge injection spikes not excessive.
Compared to the ready made AZ OPs the switches are usually not as well matched and not that well fit for the purpose. On the other side a DIY chopper can use a relatively low chopping frequency and larger coupling / filtering capacitors that are not available on a chip. A lower chopping frequency gives less current noise and a JFET based amplifier can have an 1/f noise limit lower than the CMOS process used with the AZ OPs. Especially DIY an extra adjustment, e.g. to minimize charge injection, is also possible.

For my purpose a simple AZ OP was good enough - they are not that bad and could make a pretty simple system if the source is low impedance.
So the low noise chopper is at hold for the moment. I want to get the meter part working fírst.

Chopping works pretty well also to very low frequencies. The slight residual 1/f noise one sometimes sees with AZ OPs seems to come from a limited gain of the chopper part and than relatively high 1/f cross over of the main amplifier to start with. A seprate build amplifier can be better in this respect. The main limit are thermal effects.

[David Hess]

That is pretty standard stuff for low impedance measurements.

Down to 3 mHz?

Absolutely, which is why good chopper stabilized amplifiers have a 0.01 Hz (or DC) to 1 Hz or 10 Hz noise specification.  Low noise precision amplifiers do not usually bother going down to 0.01 Hz or DC because flicker noise dominates and it would be a misapplication of the part.

Ultimately the lower frequency limit is determined by the integration time of the measurement.  If you want to measure 0.1 Hz noise, then the measurement must last 10 seconds.  For 0.01 Hz, that becomes 100 seconds.  And that is why noise at these low frequency is important for low noise measurements, and why flicker noise which increases as the frequency decreases is such a problem.

Back when I was measuring noise of my low noise differential preamplifier which used a pair of LT1028s and an LTC1151 dual chopper stabilized amplifier, I used a stopwatch to time 10 seconds of sampled measurements.  The resulting 0.1 Hz to 10 Hz spot noise measurement matched the integrated noise curves of the LT1028 and LT1151 exactly.  Noise was low enough that I could measure low values of resistance from the Johnson noise of the resistor.  I found this rather unsettling but God never struck me down for it.

[Kleinstein]

Noise was low enough that I could measure low values of resistance from the Johnson noise of the resistor.  I found this rather unsettling but God never struck me down for it.

Seeing the normal Johnson noise is one part, looking for the resistor excess noise (which is additional 1/f noise of the reistors when used with voltage) is another part where a good low noise chopper can help and it also needs to be low current noise. It is a bit unsettling when a supposedly precision resistor fluctuates.
There is no absolute need to do this with DC, but the low frequency way is closer to the definition.

[David Hess]

There are Choppers based on LDRs, but due to the resistance of the LDRs the noise tends to be relatively high. So this is more like the 5-10 nV/sqrt(Hz) level and higher. There can still be some effective charge injection from cross conduction, as the switching is slow.

There are nV amplifier based on JFET switches and a transformer at the input - these can get really low in noise (well below 1 nV/sqrt(Hz), but charge injection needs a carefull trim to get it low. This could be a way for a DIY solution - it is old style, but relatively simple. AFAIK Keithley has such an amplifier for the K2001/2002. TiN can probably tell more on these.

I wonder if a varactor amplifier like the Philbrick P2 or Analog Devices Model 301 could do it.  Varactor operation removes flicker noise but there would need to be some way to cancel drift.

[David Hess]

Noise was low enough that I could measure low values of resistance from the Johnson noise of the resistor.  I found this rather unsettling but God never struck me down for it.

Seeing the normal Johnson noise is one part, looking for the resistor excess noise (which is additional 1/f noise of the reistors when used with voltage) is another part where a good low noise chopper can help and it also needs to be low current noise. It is a bit unsettling when a supposedly precision resistor fluctuates.
There is no absolute need to do this with DC, but the low frequency way is closer to the definition.

I knew I was limited to low values of resistance consistent with the input current noise, and since I knew the amplifier's input noise and the test resistance, I verified that the added noise was consistent with the Johnson noise.  I was using wirewound and metal film resistors so I did not expect any excess noise and indeed, I never saw any.  Unfortunately I did not think to test any low value carbon composition resistors.

My design was not quite up to the nanovolt resolution being discussed but it was within shooting distance and could have been improved, especially today with better chopper stabilized amplifiers and a discrete input stage.

[branadic]

Absolutely, which is why good chopper stabilized amplifiers have a 0.01 Hz (or DC) to 1 Hz or 10 Hz noise specification.  Low noise precision amplifiers do not usually bother going down to 0.01 Hz or DC because flicker noise dominates and it would be a misapplication of the part.

0.01 Hz is an order of magnitude higher to the given corner frequency of 3 mHz in the paper, while "m" means milli, that is 0.003 Hz.

My R9211 goes all the way down to 10 mHz, but is pretty much limited by its own 1/f-noise, which makes it impossible to characterize ultra low frequency noise of voltage references.

-branadic-

[David Hess]

Absolutely, which is why good chopper stabilized amplifiers have a 0.01 Hz (or DC) to 1 Hz or 10 Hz noise specification.  Low noise precision amplifiers do not usually bother going down to 0.01 Hz or DC because flicker noise dominates and it would be a misapplication of the part.

0.01 Hz is an order of magnitude higher to the given corner frequency of 3 mHz in the paper, while "m" means milli, that is 0.003 Hz.

My R9211 goes all the way down to 10 mHz, but is pretty much limited by its own 1/f-noise, which makes it impossible to characterize ultra low frequency noise of voltage references.

That is right, but manufacturers are not going to test even lower because of the time it takes.  The DC or 0.01 Hz specification is sufficient.  Chopper stabilized amplifiers have flat flicker noise, or at least it is suppose to be flat.  If someone cares about this then they need to qualify the parts themselves which should not be too taxing.

[dietert1]

Zeroing for voltage reference comparison should be at multiple time scales. The usual time scale for low frequency noise specs of 0.1 to 10 Hz can be covered by the DMM Autozero. For longer time scales one can use one zero channel of a low thermal EMF relay multiplexer.
The 2182A is meant for high speed low noise measurements. If i remember right, a 2182A drifts about 20 or 30 nV, so a relay MUX can help there. We also have one of the old relay chopper nanovoltmeters (Keithley). It is stable to 1 or 2 nV, but of course it won't be running for months or years.
A cheap and low power consumption setup i am using has 2 HP 3457A voltmeters in parallel and works quite well when combined with MUX zeroing.

Regards, Dieter

[Kleinstein]

The varactors usually have a thermal drift about comparable to a normal diode / PN junction. So getting rid of thermal dirft is prettty hard. The varactor amplifiers are more a thing for low bias and the option to have them isolated from the rest. I don't think they are very low noise either.

It does not even need carbon resistors to see resisor excess noise. To see the excess noise one needs to apply some voltage, so it is not allways there.
Even some thin film resistors show it in a measureable amount, if one looks at low enough a frequency. I did a test to confirm low noise of a good type, but forgot to test the ones I highly suspect to show more noise, because they showed extra 1/f noise in another circuit.

I just used an AZ OP, even a chep one (MCP6V51) and it does perform reasonable well. It just needs time to measure at 10 mHz or so.
Some of the modern AZ OPs are pretty good. They may still show some very low frequency 1/f noise, but even that is often no to bad and it is hard to tell appart from thermal effects of the setup. Thermal EMF and temperature fluctuations can also cause some 1/f noise.
There was another thread on AZ OP testing.

[guenthert]

[..]
Nanovoltmeters are great to intercompare multiple references with the 3-cornered had method. But they are expensive, a used 2182A is almost the price for a new one.
[..]

     The current generation is fairly expensive, but I've seen a bunch of Keithley 182 (as recently as 2014 still good enough for the Laboratoire national de métrologie et d’essais [1]) for a quite reasonabe price on e-bay.

     The old not-quite-nanometer Keithley 181 can be had for little money (the original input cable would double its value 

     When going the DYI route one mustn't underestimate the effort in getting the mechanical construction (RFI shielding / minimizing the effect of thermal EMF) right.

[1] https://www.bipm.org/documents/20126/47750471/EURAMET.EM-S41.pdf/39f8462f-4bb8-09ef-67d1-661535764a36

[David Hess]

The varactors usually have a thermal drift about comparable to a normal diode / PN junction. So getting rid of thermal dirft is prettty hard. The varactor amplifiers are more a thing for low bias and the option to have them isolated from the rest. I don't think they are very low noise either.

Bob Pease reported the same thermal drift that you would expect of a good discrete design, which of course the P2 was, but low frequency noise better than commonly available.  Something would have to be done to fix the thermal drift; I was thinking of chopping the varactors but that defeats the purpose of using them except for their virtues which you identify.

Some of the modern AZ OPs are pretty good. They may still show some very low frequency 1/f noise, but even that is often no to bad and it is hard to tell appart from thermal effects of the setup. Thermal EMF and temperature fluctuations can also cause some 1/f noise.

The above point cannot be overstated.  Noise from thermal EMF will overwhelm low frequency noise of even non-chopper precision amplifiers unless careful design and construction is used.

[e61_phil]

I've seen a bunch of Keithley 182 (as recently as 2014 still good enough for the Laboratoire national de métrologie et d’essais [1]) for a quite reasonabe price on e-bay.
[1] https://www.bipm.org/documents/20126/47750471/EURAMET.EM-S41.pdf/39f8462f-4bb8-09ef-67d1-661535764a36

The question is what do you want to measure. For a 10V comparision a Nanovoltmeter isn't really necessary. In many old publication they used a Fluke 8842A 5.5 digit meter with 100nV resolution. That is 0.01ppm at 10V and should be fine. The resolution may also be increased with averaging, especially if you average over 100s.

[iMo]

I wonder, myself, what would be the practical limit when comparing two references with that "simple single opamp" null-detector/meter and a 34401 as the meter. I messed with it years back, a chopper opamp (LTC1050 I think with 1000x amplification). With 1mV resolution dmm (at that time) I saw ~1uV diffs, imho.

[Kleinstein]

For a Nullmeter you don't even need a 34401 or similar full fledged meter for dispaly. Just a simple ADC (e.g. ICL7106 for a dispaly) or even an µC internal one is sufficient. The scale does not have to be very accurate if it is only about a small residual different or just to adjust to zero. So the ADC and refrence part is not that importand and I would consider something like a MCP3421  (18 bit SD with internal reference in SOT23-6) sufficient for a null meter and it is easy to power from battery.  There are other similare chips too.

The point with chopper OPs is, that one may need some additional filtering at the input and some protection. With just the OP input, the AZ inputs can reacto to the source impedance in the MHz range. So things like cable inductance and parasitic capacitance can effect the offset and that is not what you want from a meter. So it  needs some extra filtering to suppress this effect.
Here it is not about seeing 1 µV differences, but more like 100 nV relatively fast and 10 nV with a little time and ideally even 1 nV after averaging over longer time (e.g. 100-1000 seconds). I would consider a meter worthy nV meter if you get the noise down to the 20 nV/sqrt(Hz) range and the stabilit over a an hour to better than 100 nV, preferrably the 10 nV range. The LTC1050 OP is still a bit noisy. AFAIK the Keithley 2000 uses this as it's input and it should be the main noise source in the 100 mV range. So to really call it a nV meter it would need a slightly lower noise OP.

The other point is than to not get much other errors from thermal EMF and from the protection.  The simple protection with a resistor in the 100 K range (e.g. like used with the 34401 and other meters) would add allready significant noise, and in combination with a bias in then 100 pA range also quite some offset. So the protection usually needs to be more than just a resistor and clamps at the input.

[Mickle T.]

It was my old quick-and-dirty project of micro/nanovoltmeter for academic research lab. Based on reversed and totally reworked Yokogawa chopper DC preamp.

https://youtu.be/xlaAz28XRSQ

[MegaVolt]

Design of ultra low noise amplifiers.pdf  (figure 5)

[RandallMcRee]

The circuits posted so far all feature nanovolt sensitivity but differ in some important respects that create traps for the unwary. I know that most (all) of the posters know these things but for the benefit of my past/future self here are the important distinctions:

1) Is it AC or DC coupled?
  To put it simply AC coupling makes for an easier circuit but, as should be obvious, is quite restrictive. E.g. you cannot measure the difference between two 731Bs as you can with a nanovoltmeter.

2) Is it chopped or not?
  Chopping generally deals with 1/f noise admirably but has other tradeoffs with poor voltage noise, current noise (or both).

3) If it is not chopped then, typically, you can still use other techniques to overcome the 1/f noise. So, for example, here is an instrumentation amplifier with good nanovolt sensitivity:
    https://www.analog.com/en/analog-dialogue/articles/low-noise-inamp-nanovolt-sensitivity.html
  You can then use so-called bridge excitation techniques (LT AN-96) to remove the IA amplifier offset. That can be as easy as multiplexing the two inputs and averaging.

4) Along with such details as Thermal EMF there is always gain drift and other resistor TCR problems that are not often mentioned (so another trap for the unwary). The AD8428 mentioned above, for example, is exemplary not just because you can build a 0.7 nV/√Hz IA but also because the precision resistors are embedded in the IC. Many of the circuits shown so far, cannot realize their full potential without buying 0.2ppm (say) matched resistors or resistor networks.

Let me know if any of the above is misleading/wrong.
Thanks!

[MegaVolt]

AD7195  RMS noise: 8.5 nV at 4.7 Hz

[MegaVolt]

ADS126x  Noise: 7 nVRMS 2.5 SPS

[guenthert]

I've seen a bunch of Keithley 182 (as recently as 2014 still good enough for the Laboratoire national de métrologie et d’essais [1]) for a quite reasonabe price on e-bay.
[1] https://www.bipm.org/documents/20126/47750471/EURAMET.EM-S41.pdf/39f8462f-4bb8-09ef-67d1-661535764a36

The question is what do you want to measure. For a 10V comparision a Nanovoltmeter isn't really necessary. In many old publication they used a Fluke 8842A 5.5 digit meter with 100nV resolution. That is 0.01ppm at 10V and should be fine. The resolution may also be increased with averaging, especially if you average over 100s.

     Well, in old publications they used a galvanometer. 

The measurement will be noise limited.  I couldn't find noise figures for the 8842A, but it's 24h accuracy is specified as 50ppm of reading + 20 counts in the 20mV range, which one can take as a hint.  Nanovoltmeters fare better (the old Keithley 181 can be expected to be an order of magnitude better there).  But given the noise of the sources, there might be little to gain.

[guenthert]

For a Nullmeter you don't even need a 34401 or similar full fledged meter for dispaly. Just a simple ADC (e.g. ICL7106 for a dispaly) or even an µC internal one is sufficient. The scale does not have to be very accurate if it is only about a small residual different or just to adjust to zero. [..]

      Quite so.  For a true null meter (error detector) the scaling error is of little importance (and the offset one can calibrate oneself easily).  Comparing then two voltage sources gets awkward however, if they are stable, but not quite close.  One can use the potentiometer method (using a third source) to determine the difference, but that's a bit inconvenient (Bureau of Standards used long time ago a Brooks comparator for that).

[Kleinstein]

For a nV meter the ADC noise is the less important problem. It still help if the ADC is continuously sampling the input and not just reading the input half the time and do an internal zero the other half. This would double the time needed because it increases the effective BW for the input noise and amplifier noise.

So there can be an advantage in using an SD ADC (they usually do near 100% sampling) and not an ADC like the ICL7106 that does only some 25% sampling. The HP34420 (otherwise a good nV meter) is in this respect not ideal.  The HP typical concept with 50% input sampling and 50% zero is not the best choice for a nV meter.

The noise of the source is another point. Low BW and good filtering to keep the noise BW for the source noise may be more important than a super low noise amplifier, when it comes to just comparing 2 higher voltage references.  There are also cases where the source is low noise (e.g. checking thermal EMF) though.

Modern choppers don't have necessary high voltage noise. This was a point in the past, especially when doing only one sided switching (e.g. signal and zero) instead of polarity switching.

The right balance of voltage noise and current noise depends on the signal impedance and this can vary quite a bit.  When the nV range matter the sources are usually relatively low impedance, as otherwise the resistor noise of the source would become significant. So the current noise may not be that important for a nV meter than it is with a more normal voltmeter good for µV resolution. Anyway there is not 1 nullmeter to fit all.