@intabits: Have any of the reference boards you've bought deviated significantly from each other? I'd just like to be able to calibrate my 4.5 digit DMMs well enough that I'm within 1% for general lab use. Maybe the typical reference is good enough for that without a cal certificate?
Sorry, didn't notice that this was directed at me.
At least for the units I have, I'd say they should be easily good enough to get a
4.5d any DMM to within 1%.
As to deviations, I haven't done much with mine except for this:-
Which is a mess that I've been running for year now.
It's made almost entirely out of ebay crap - I just wanted to get something logging away...
It consists of:-
1/ Two large hot-swappable LiFePO4 battery packs, powering:-
2/ An Arduino UNO with a data logging shield (RTC & SD card).
3/ A DS18B20 temperature sensor, plus a DHT11 (temp and humidity sensors)
4/ A REF5040 providing 4.096V to:-
5/ A cheap ADS1115 module (4Ch 16 bit ADC), which converts the output of:-
6/ 2 of the AD584 modules
7/ 2 eBay "LM399"s (outputs are divided by 2 for the ADC)
It logs timestamp, 2 temp, 1 humidity, and 4 VRef readings every 10 seconds (way too often, but storage is cheap).
It's sitting in my back junk room, where it collects a lot of dust, and is well exposed to the elements (near an open window, and a doorway that is almost open to the outside)
Below is the full year output (DS18B20 temp and the VRefs). There are a few glitches, and a month of missing data that I'm still kicking myself for losing.
Obviously, my software also needs a *lot* of work.
The strong upward trend (and a lot of other variation) I put down to the crappy ADC. But it can be seen that the 4 VRefs track each other quite well, and the two AD584s are nearly always with an LSB of each other.
This can be seen better here, which is 10 days in the middle of the above.
At some stage, I'm hoping to do some analysis of all this, trying to see what correlates with what. Next version will use a better ADC (and maybe two, with one kept further away from the environmental changes)