Electronics > Metrology

How I made my nanovoltmeter

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jaromir:
it is nearly 256 days since TiN released his call for nanovoltmeter designs [1] and it trigerred quite a few responses here [2].
I tinkered with DIY test gear before, but dedicated micro- and nano-voltmeters were always too niche and mysterious for me, so this contest didn't resonate with me too much at first. In fact, I never operated any nanovoltmeter before, had zero practical experience in low-level voltage measurement and it took me a few days to pluck up the courage for this voltnuttery area.

I will not repeat all the stuff I already wrote into public repository here [3]. so perhaps just small recap of the design process. As usual, the major starting problem was getting single piece of metal enclosure of right size that doesn't cost an arm and leg. I found some enclosures at Modushop [4], but it still needed some minor adjustments, like workarounding weird decision of using self-taper screws.
Electrical design was centered around finding good low noise amplifier. I went through half a dozen of test designs to finally settle on paralleled MCP6V51 opamps as good compromise between price, parameters and "time to market" (which is always major constraint in contests). The rest of analog circuitry wasn't that complicated - main amplifier with switchable gain, plus few MUXes and relays to connect the respective amplifiers where needed. I implemented automatic calibration (ACAL) which is not that complicated for voltmeters - it takes resistor divider string and way to attach one of divider outputs to amplifier input. Assuming good ADC linearity and known one voltage on the string, ACAL algorithm can measure voltages by more than one measurement range and calculate gains and offsets of amplifiers. One those are known, each range gain and offset can be adjusted.
ADC is of integrating charge balance type, with now rundown and residual integrator voltage readout. I could have perfectly used integrated sigma-delta ADC, but this was more fun.
One of funny aspects of this design was PSU - at first I was aiming at 50Hz dual back-to-back transformer type, but halfway through I realized I can feed the transformer by audio amplifier and local oscillator. That enables both DC power operation as well as AC line power (by using appropriate AC/DC power module).
Also, this project is GPIB playground. I never implemented any GPIB device, so I thought like this is good opportunity before this interface slowly fades into history. Unfortunately, deadline approached earlier than I expected, so I postponed it for later. But ethernet interface works and talks via SCPI-like commands.

The major design goals set up by the contest - 30nV p-p noise@10Hz, nV/10V combined inputs and two switchable inputs - were met and I'm happy to learn a thing or two, with byproduct of gaining an actual nanovoltmeter (NVM). It could be improved by many ways, a few major TODO points are also included in the repository and I'll try to address them as soon as possible.

While working on this project I realized it's probably quite complex and I really felt like it could be done in simpler way, albeit with less stringent design criteria. I cherrypicked some parts of this NVM and integrated then into much cheaper and simpler instrument. I really hoped to finish it until deadline, so I can present both full blown and budget versions of NVM, but unfortunately I had to focus more on the large one, so NVM junior has to wait a bit. Surely I'll share the design publicly and announce it here, too.

Attached are pictures of a whole device, short term noise measurement and 1uV steps provided by Keithley 260 source. Also, you may take a look into imgur album with random photo dump [5]


[1] https://xdevs.com/article/nvm_comp/
[2] https://www.eevblog.com/forum/metrology/nanovolt-design-challenge-build-and-show-your-own-nv-meter-in-256-days/
[3] https://github.com/jaromir-sukuba/nvm
[4] https://modushop.biz/site/
[5] https://imgur.com/a/fnK1Evj

macaba:
Great work Jaromir, you're an inspiration to us all.

RoGeorge:
Wow!  :-+

mawyatt:
Superb work, and stunning effort  :clap:

Really like the way you approached this task, the selection of the MCP6V51, and clever use of audio amp to drive the power transformer  :-+

Please post more details and images if you can, and thanks again.

Edit: Forgot to ask if you noticed any artifacts from the chopping action of the MCP6V51? Since you are using 100 devices, do they show a subtle beat against each other, or maybe injection lock to each other? Didn't notice the chopping frequency mentioned in the data sheet, but appears from some of the graphs to be ~50KHz.

Best,

DH7DN:
Congrats for completing the challenge. Thanks for sharing! ;D

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