Nvm

Please note this nanovoltmeter design was superseded by a new and better design, which will go public in this Github repository. Soon(TM)

What is in this repository?

Here is the complete set of design files, manufacturing data, source files, test results and other resources that I used in the process of building a nanovoltmeter. This process was not a coincidence, but a response to the nanovoltmeter challenge by TiN on his website

What is a nanovoltmeter and what is it good for?

Well, a nanovoltmeter is a voltmeter that has the sensitivity, noise and stability to resolve voltages down to the nanovolt. While many multimeters do have quite sensitive low voltage ranges, the resolution (least significant digit) is usually in the order of hundreds of nanovolts - and value of this last digit is often diminished by the noise and nonlinearity of the meter. So, for proper low signal measurements, having a higher sensitivity is mandatory. Measurement of such low voltages is useful for many applications, for example resolving small voltage differences in various differential and bridge circuits and the measurement of very small resistances - with an appropriate current source. Commercial nanovoltmeters do exist, like the Keithley model 2182A or Keysight 34420A, but with a hefty price tag, so a DIY design definitely has its place.

xDevs challenge

While I have been tinkering with DIY test and measurement tools for a while now, I never had an impulse to discover the nanovolt world. Such a trigger happened on September 2nd, 2021, when TiN from xdevs site launched the nanovolt challenge. In nutshell, it called for the design of an open-source nanovoltmeter, with parameters comparable to commercial designs and a timeframe of 256 days. That is a non-trivial task, and in my case it was even more so, because I had not only never designed low signal circuits before, but had I never operated a nanovoltmeter. That is quite an unfavorable starting position and jumping into the deep end rather than dipping my toes into the problematics. To ease my mind, I waited a few days to see how others reacted on the eevblog forum. I can't say that it encouraged me. On the other hand, I spent a few days researching online sources and studying the service manuals of proven test gear from reputable vendors. Too bad they stopped supplying us with schematics in early 90s of the last century; but even that material was very helpful in understanding both what the requirements are for such a meter, as well as the means to achieve it.

NVM design, part 1 - design requirements

Armed with new knowledge I started dissecting the challenge requirements.

Have local onboard power regulation. Single common DC (+9 to +24 VDC) or 110/220VAC mains input jack is expected.

Okay, I think I can do this. On the other hand, any reasonable mains-powered voltmeter has to have an isolated PSU section which powers the input dividers/amplifiers and ADC, so that the measurement potential could be unrelated to the digital section with all the user and communication IO - which is needed to be earthed or somewhere close to that potential. This one calls for either DC or AC power, but it could be combined by having a PSU operating from an appropriate DC voltage (12V sounds like a good starting point) and attach a small AC/DC SMPS to provide power from mains.

Provide DC Voltage measurement ranges ±100 µV or below and include ±1V and ±10VDC range.

This one looks innocent, but having both 10V and 100uV input on the same jacks implies either switching input amplifiers, or inserting a voltage divider into the signal path, preferably through a relay. Some older nanovoltmeters (Keithley model 181) had two different inputs - one for higher ranges, another one with low TEMF connector for sensitive ranges.

Have at least two user-accessible input channels for signal to be measured.

Oh my, another relay. Relays are good, but may introduce offset errors into signals due to thermal voltages (TEMF, short for Thermal Electromotive Force) generated on the contacts, when the relay heats up from coil power. The voltages may be around a few microvolts down to nanovolts, definitely something to consider in this design. TEMF is the main enemy of the nanovoltmeter and this acronym will be used in following text a few more times.

Have low-thermal connection interface to minimize thermal EMF parasitic errors.

TEMF again, I told you so.

Provide at least 5½-digit resolution for each reading.

Sounds reasonable. I've done long-scale ADC projects like voltmeter and voltohmmeter so perhaps I could recycle something and also bring something new to the table. I think commercial sigma-delta A-to-D converters would do the job too, but a DIY ADC definitely has more to it.

Ability to digitize input DC signal with resolution at least 10 nV and noise better than 30 nV peak to peak over at least 0.1-10 Hz bandwidth.

More numeric requirements, nice. 30nVp-p in 10Hz bandwidth is not a very relaxed requirement - older meters like Keithley model 181 would not cut it, current models so-so.

Have autozero functionality to correct for static offsets.

That is a logical requirement. I will put in some more effort by introducing autocalibration.

Have galvanic isolated analog front end, with isolation resistance to earth/chassis better than 10 GΩ.

That is what I proposed earlier, too.

Device should have ADC (any type) integrated.

This is likely to frame the project into standalone modus operandi.

Have good long-term stability and use ovenized DC voltage reference (LM399, LTZ1000 or LTFLU with oven).

I wouldn't think of using anything worse than an LM399 (or ADR1399) anyway.

Provide RJ45 Ethernet and/or IEEE-488 GPIB interface for communications with external world / external equipment.

Fair point. USB is simpler, though.

40W total input power budget (friendly to battery operation for sensitive experiments)

You wouldn't want your nanovoltmeter to consume more than 40W anyway. 40W is quite a bit of heat and it's not easy to dissipate it without introducing more TEMF errors.

Device should be fully operational as standalone device (e.g. no debuggers or external equipment attached to make it work).

OK, fair.

After a bit of optimization, I came up with this block diagram. It's basically done to fit the requirements above, with a few spicy parts I added for extra fun. Input from either input connection is selected via an input multiplexor. There is also a third input, to be discussed later. This MUX passes the selected channel to a low noise amplifier (LNA) to be amplified and then routed via another MUX into the main amplifier. whose output is brought to the ADC. This ADC has a nominal 10V input range, so that the basic range with LNA is 10mV (10mV x 1000 = 10V). In order to have even more sensitive ranges, the main amplifier has option to switch to x1 (pass-through), x10 and x100 gain, combined into additional 1mV and 100uV ranges. When the LNA is bypassed, the input signal is brought directly to the main amplifier, allowing for 10V, 1V and 100mV ranges, seamlessly covering 100uV to 10V input range. That would make a nanovoltmeter already, but I added two more blocks - first is easy, a low-pass filter (also known as LPF) to cut down noise, especially on sensitive ranges. Another one is an auto-calibration block, which serves to decide the gain of both amplifiers. Ideally, the meter needs only to know its onboard 7V reference voltage and can derive lower ranges automatically without calibration/adjustment and keep it precise after resistors in dividers and amplifier drift due to age or temperature. The entire upper portion of the circuit is held behind an isolation barrier in order to keep the input terminals separated from the power supply potential - either external DC or mains voltage. The bottom section consists mainly of the PSU and digital circuitry, including user IO and communication channels, like Ethernet and USB. For the added fun factor I also included GPIB as a low priority subproject.

NVM design, part 2 - divide and conquer

Designing such an instrument, especially on a DIY base, is a lot of pingponging between mechanical and eletrical design. I could imagine how large the thing would be (say, an A4 page size as a footprint) and I knew I had to cram everything inside. Choosing a larger enclosure would give me much more freedom and other benefits (like easier dissipation of device heat, helping fight TEMF), but would be impractically large. Finding an appropriate enclosure wasn't easy - at first I thought of using G756 I used before in my DIY SMU, but I preferred metal enclosures here - because of shielding from electrical interference, and with a steel enclosure even a bit of shielding from magnetic interference. Metal also helps with heat dissipation. Finally I settled on the 1EP802825 from Modushop. That was the right time to take a look at the electrical domain. The Earthy and floating parts (separated by an isolation barrier) have to be physically separated in sub-enclosures (that calls for two PCBs minimum) and in order to have easier debugging and modification of the circuit, I decided to separate the floating part into two PCBs. One would hold FPGA control, the reference and ADC, while another one would consist of both amplifiers and ACAL circuits. Since the enclosure is 80mm tall, there is no problem