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.

I’ve connected the reference to all three 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).

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
Fluke45 2.050 4.102
Amprobe 37XR-A 2.047 4.098
ina260 breakout 2.052 4.103

The good news is that the Fluke and the Amprobe are within the reference’s tolerances at the 2.048v level, and the Amprobe is still within reference at 4.096. The Fluke though, well, its calibration sticker says 2001, so it might need some TLC.

The ina260 is within it’s stated 1% tolerance, indeed, it’s nearly as good as the other two with much better ratings.

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, probably the Siglent SDM3045X. This thing is rated to be 0.01% accurate which is about twice as precise as I need. Perfect, it’s been weeks since I had a new toy.