Nanovolt design challenge - build and show your own nV-meter in 256 days [DONE]

[ch_scr]

[...]
Unfortunately, some of the really interesting FETs are obsolete these days, such as 2N5564/NPD5564 or BF862, [...]

-branadic-

Well there is LS5564 from "Linear Systems".
https://www.linearsystems.com/lsdata/datasheets/201187%20-%202N5564%20Series%20Rev%20A3%20dated%202017%2003%2003%20-%20Jaime.pdf
Sure it's not cheap and very much single source but you can (or could?) get a newly made 2N5564 equivalent.
At least about 2 years ago when I bought some for a repair. They are not advertised in the new catalog so not sure.
Even if not, they certainly have the LSK389A which is lower current but also lower noise and with slightly better matching.
I had a very good experience with their german rep "Ingenieur-Buero-Fluck" buying as an individual.

[macaba]

For the with a chopper in front the requirements for the amplifier are less stringent. The target than would be more like 0.7-1 nV/sqrt(Hz) at some 1 kHz. There are a few FETs to choose from and one can get away without good matching and possibly even with just 1 pair.
The amplifier gets even less demangind if one goes with the chopper - transformer - amplifier configuration (e.g. like the EM nV preamplifiers).

Makes sense, the main reason I'm planning on JFET input (with optional CDS mode, aka "chopper") is so I can have switchable 480uF/100k HPF before it so that eliminates most input types.

[macaba]

I have been simulating various input stages.

One concept that is novel to me is the idea of using the same input stage for the entire range from 10V to nV rather than switching between 2 parallel paths. I've come up with a solution but it has a rather high BOM cost; using discrete JFET follower (with fixed power dissipation in JFETs) on the frontend of low noise bipolar opamp (and the expensive design choices that follow from that requiring the next amplifier stage to also be low noise).

I am wondering if anyone has seen/could share successful examples of:
- Switching the JFET input stage between follower and gain configurations (i.e. switching between "resistors above" and "resistors below" and switching opamp inputs around).
- Compensating the JFET gain arrangement successfully to work as unity gain (I just can't seem to get a stable arrangement of RC compensation in simulation).

[Kleinstein]

I got at least the simulation stable also with a gain of 1. The compensation may have to be more than just simple dominant pole, but with a 2nd step somewhat lower. This is kind of needed when one has very high loop gain. The input amplifier in the R6581T could be used as an example.
One may have to switch the compensation with gain.

One could consider less current for the JFETs when using a gain 1. This also reduces the gain of the JFET part.

I think that directly switching between an long tailes pair and source followers is tricky, as the switches add possible thermal EMF, resistance and unwanted capacitance.

I like a seprate preamplifier stage for the highest gain for 2 reasons:
1) one does not need gain switching at the input and avoid the swith there
2) the FB divider can be lower resistance and sees less self heating when the output of the first stage is lower (like 100 mV).

A parallel input stage for the higher voltages mainly needs a way to isolate the low voltage input - the higher votlage input can have it's own protection and input MUX. These parts are relatively non critical.

[jbb]

... you know, it's horribly boring but the ADA4528-1 in G=+100 configuration is an option for the 100uV range:

• Vos < 3uV
• Drift < 0.015uV/C
• Noise ~6 nV/rt(Hz)
• Noise ~100 nV p-p 0.1-10Hz
• Ibias <= 600pA (over temp)

[coppercone2]

too much work. good NVA have a differential/single ended switch and a ground lift switch typically.

if you want the standard features you are going to need to figure out good switch setups. I imagined the front panel and I am already tired

and a good 'prize' that might make this worthwhile could be one of those fancy LTZ1000 boards, you know, so you have something to test, fully assembled with a known good reference.

[1audio]

I just acquired a Keithley 148 Nanovoltmeter. I have found some really useful info on how hard it is to make these. When you are measuring voltages 20 dB below any thermoelectric effects its going to be all technique to make something useful. Some references with useful info:

https://elektrotanya.com/keithley_148_measurement-of-nanovolts_nanovolt-mero_sm.pdf/download.html   A real primer on technique.

Surprisingly Tek has OK scans of the manuals for these vintage instruments. The manuals actually have a lot of useful detail, since there were from a period where mfr's shared how the instruments work.
https://www.tek.com/manual/keithley-model-147-nanovolt-null-detector-instruction-manual-rev
https://de.tek.com/manual/model-148-nanovoltmeter-instruction-manual-29029-rev

The two instruments are very similar but the 147 has a writeup on interfacing with L&N etc. precision potentiometers.

[egonotto]

Hello,

is someone working on this Nanovolt Challenge?

Best regards
egonotto

[MegaVolt]

The end of the competition on May 17?

[TiN]

MegaVolt
Yes, that's the idea.

egonotto
I haven't heard anything from anyone so far. You can be first 

[coppercone2]

its really not enough time to do such a (good) design unless you already work with those kind of designs

[macaba]

is someone working on this Nanovolt Challenge?

I think we’re hesitant to say we are working on this (the people I know of) until we know if we can meet the deadline. I’d expect to see more activity in the few weeks before.

[ferret_guy]

I have been working on and off. Got my prototype PCB back and did some noise and input current testing.

Here is a photo of the current PCB:



I soldered the 4 In-Amps on one at a time and measured their performance, here is a (offset corrected 0.1-10Hz filtered 100sec) plot of the PDF of the shorted inputs:



Some statistics:

	1 In-Amp	2 In-Amp	3 In-Amp	4 In-Amp
RMS (nV)	6.76	5.04	4.59	4.25
Pk-Pk (nV) (10sec) average	40.05	32.33	31.40	31.46
Pk-Pk (nV) (10sec) max	48.66	47.07	47.12	55.43
Pk-Pk (nV) (10sec) min	29.63	24.48	22.64	18.41

It is pretty clear to me that adding more than 4 In-Amps is a losing proposition.

I additionally did some testing, just leaving the board in a box, sandwiched in a towel to prevent air movement, and recorded the warmup and thermal drift over time, over a 5C change in air temp, I saw a 500nV change in the shorted voltage:



I did some testing of the input current, between +-900mV it is about 25nA (flowing into the input). As I approach +-1V the input current hits 3mA, which was surprising (though in retrospect not that surprising). The datasheet only mentions limits in the Absolute Maximum Rating section (which of course I failed to read), so my elegant nV to 10V input design without any switching (except for in the ADC) will not work as planned.

[RandallMcRee]

Is that design based on ganged AD8428s? If so, note that pin 7, is only ever connected on one IN-AMP.

https://www.analog.com/en/analog-dialogue/articles/low-noise-inamp-nanovolt-sensitivity.html

[dietert1]

Don't forget that 10 nA input current times 1 ohm probe cables = 10 nV.

Regards, Dieter

[coppercone2]

Don't forget that 10 nA input current times 1 ohm probe cables = 10 nV.

Regards, Dieter

1 Ohm probe cables?? that should only be the case for the most strict cryogenic setups (i remember something about weird not so conductive metals being used at these temperatures).. something is not right if your probe is 1 ohm unless its a insane setup.

[Kleinstein]

1 Ohms for the cable, switches (to get 2 inputs) and the signal source are not that unreasonable. The usual thermocouple probes are in the 10 ohms range, somtimes more.

Another part that can add some resistance is the protection of the input agains overvoltage and possible EMI (though AFAIK not directly part of the requirements in the challange).

Several nA of input current are definitely a problem for practical use, though it may still be fit for the conditions of the challange.

[coppercone2]

oh, for thermocouples that changes the situation completely. I guess you would need a compensation junction external to the meter right? I did not think to hook up a thermocouple to one. I never saw the internals of a nanovoltmeter from HP that has a thermocouple button on it.. do they put a reference junction inside?

And you are right for a nanovolt meter you can put protection on it, unlike a electrometer, since you are measuring low impedance sources.

I was imagining switches like a resistance standard for a good nanovolt meter, silver triple bladed... and I would go for heavy wire chokes here, not a minimum size SMD inductor)

[KT88]

A nanovoltmeter is a slight overkill for TC applications (several uV/K). But even then the offset could be calibrated out to the most part.
My concern would be that it might damage or at least disturb a Weston Cell if someone happens to still use it.

[Kleinstein]

The low voltage on thermocouples are still a challange to normal DMMs and so you would want one with good sensitivity. So it would make some sense to have cold junction compensation on a sensitive voltmeter - though this mainly makes sense with an dedicated input, with a proper plug. There are other sensors that use thermocoules to measure differences directly.

There are other sources that are not that low in resistance. Due to the Johnson noise, there is however less need to care about source resistance in the higher kOhms though. A meter should still have brought range of uses, even if overkill in some ranges.

The difficult part is the protection - this tends to add more like 10s of ohms. Even a low current (e.g. 100 mA) fuse may be a few ohms.

One can compensate for the average input current. This is done with some meters, like the HP34420, some fluke and datron meters with BJT based input.
However it is very hard to compensate for drift and noise of the input current.

[KT88]

Input protection is a different topic than effects related to input bias currents...
Input protection has to be designed to a given spec - which determins the effort one has to put into the protection.
There are two main strategies that can be combined arbitrarily: current limitation and voltage limitation.
In the usecase of ultra-low voltage measurements a current limiting impedance can in fact cause problems. In this case it is possible to use a cascaded voltage limitation with a first set of anti-parallel diodes against a guard (with a low voltage difference to the input) and a second voltage limiter against the supply rails or transil diodes. An example for this approach is the ADA4530. One has to be aware that the source is the current limiting element in this case.
A trap to avoid is the choice of diodes that have a package that is transparent for IR or visible light...

[Haasje93]

I have been working on and off. Got my prototype PCB back and did some noise and input current testing.

Here is a photo of the current PCB:

Hi!

First off all, i want to say that i am following this thread with interest. I am learning new stuff am i am looking forward to what everyone has come up with.

By seeing the above picture i saw something that was not as it is supposed to look. The capacitors below the 4 amplifier, the solder joints don't look right to me.
And it look like there is a short on the left IC above the raspberry pi.

Kind regards,
Christiaan

[egonotto]

Hello,

push!

46 days, I am curious whats come.

Best regards
egonotto

[RoadDog]

I’m interested to see what folks are going to come up with for this as well. Been quiet though so not sure how many folks are actively working on it.

What’s next TiN? A millOhm meter?

[TiN]

I haven't received any projects or WIP items from anyone, so I believe this contest is a flop at this point.
But who knows, maybe there will be 10 projects published on last day.