(Updated with info on the Siglent)

A voltnut, or "metrology enthusiast" as our Brit and DownUnder friends say, is a person who gets really excited about knowing to many decimal places how much voltage and current is running through our little devices. I don’t really need all that, but when I’m playing with digital-to-analog conversion, I do need to know pretty accurately what the real analog voltage is.

I’d like to know to the millivolt, which at 5v means 0.02% accuracy, or "4 1/2 digits," in metrology-speak. Turns out that’s about five times more accurate than anything I have now.

This Fluke was built while I was in college.

I’ve connected the reference to all threefour of my measurement devices. First I have an ancient but wonderful Fluke 45 multimeter which claims precision to a tenth of a millivolt at the medium reading speed at 3v, or 0.02% (I can’t quite tell from their table). It has a calibration seal dated in December, 2000.

The newest toy on my bench is a Siglent SDM3055 multimeter. I won’t say more about it because mjlorton at YouTube has done a great review. The calibration cert on mine is good thru May 2020.

The Siglent multimeter is gorgeous

Next is my 10-year-old Amprobe 37XR-A, a beefy tool for serious electricians. This beast has been to Burning Man about 8 or so times, but it cleans up nicely for lab work.

Finally I’m using Adafruit’s ina260 voltage and current sensor which they report as "better than 1% accuracy."

In this test, I’m using Adafruit’s Precision LM4040 Voltage Reference Breakout. I give it about 5v, and it puts out 2.048v or 4.096v at 0.1% accuracy. That means +/- about 2mV and 4mV at the two output voltages, respectively. Setup in the pic below.

Not to put too fine a point on it, I’d like to know which of these devices is the most accurate. Alas, this test doesn’t really tell me. The results are in the table below.

device 2.048v 4.096v
Siglent SDM3055 2.0503 4.1032
Fluke45 2.0506 4.1036
Amprobe 37XR-A 2.047 4.098
ina260 breakout 2.052 4.103

The good news is (i) the calibrated Siglent and the Fluke are indistinguishable at 4.096v, and nearly so at the 2.048v reference (I believe the Siglent). They vary by less than half a millivolt here and in other tests. (ii) The Amprobe and the ina260 are within their tolerances at the 2.048v level, and the Amprobe is still within reference at 4.096.

The ina260 is within it’s stated 1% tolerance, indeed, it’s nearly as good as the other two with better tolerances. This said, it is not a voltmeter: it’s a very delicate IC on a breakout board. Reverse the polarity and you’ll fry the IC, and you can fry an upstream microcontroller’s I2C bus connected to the ina260 board. How do I know this? you wonder.

All this said, I can’t tell from this test which is the most accurate because most of the variation is within the variation on the LM4040. I also bought a AD584 reference, but it produced arbitrary values and seems to be defective.

Three action items emerge: (1) replace the AD584; (2) get the Fluke calibrated. And (3) indulge in a fancy new DMM, which I did. It’s great.