@Mark: I started off congratulating you on the elegant and KISS design of your little calibrator project but I can't help thinking you're trying to put me off copying it now! As I said before, I like the idea of a unit whose accuracy can be assured without recourse to traceable calibration in a lab. You introduced me to the appealing concept of combining several components of one tolerance to achieve a higher tolerance. I shall be happy enough if the accuracy is slightly compromised when trying to calibrate Avominors as I want it primarily for DMMs and maybe oscilloscopes, minimum input resistance 1MΩ. I'd ideally like a combination of the features of yours and the Time Electronics 1030.
Taking a lengthy look for some reference ICs I found some packets of low-spec devices for 2.45, 2.50 and 5.0V before I found a bag of LM399AH 7V references. Although these are only the 5% type (the better selected versions are 2%) I reckon that I can connect 10 in series to get better than 1% overall. The datasheet is on datasheetarchive where you will see that these devices are ovenised for temperature stability which will be particularly useful for us working in the back room or garden shed - the warm-up time to settle within .05% is only 3 sec.
Looking at
AN-184: References for A/D Converters shows these were intended for accurate data conversion, but the author shows how the device can be used to give a buffered output (for Avos?) with a temperature coefficient of 3 or even 1ppm/°C. I attach his suggested circuit diagram here.
While we are in isolation I had hoped there'd be plenty of time to start projects like this, but then there's SWMB Reasured Then Obeyed so it could be some time yet! Please allow me to send you a couple to play around with, I'm sure you will appreciate the spec's. There are plenty to be had on eBay, but the device markings always look very different to my NOS 1980s pieces, and we know what that usually means
