When I was young there was a sign on the wall in my grandfather’s film cutting room, it went something like this “Fred was a wheeltapper – one year Fred changed 387 wheels before he realised his hammer had a crack in it.”
Having experienced ‘wheeltappers syndrome’ a few times in the past (including with a venerable Fluke multimeter) we embarked on developing our own precision voltage reference.
We started the design by looking for a quality voltage reference IC that would have a low temperature co-efficient, low drift with aging and had a built-in output amplifier – in the end we selected the excellent Linear Technologies LT1021 IC. As the LT1021 is housed in a PDIP-8 (plastic package) we knew that drift due to humidity could be an issue, as could drift caused by mechanical stresses on the package (both can be solved by using the prohibitively expensive TO5 can package version).
The PCB design employed lowers the potential for thermally induced PCB stresses to affect the reference IC. Baking the whole PCB assembly at 125C for 24 hours and then coating the reference IC, associated components and the adjustment trimmer in more than 5 layers of silicone conformal resin helped reduce sensitivity to humidity changes.
Drift due to aging – assuming that you are able to keep the temperature, humidity, air pressure and supply voltage constant, the main contributor to drift will be aging of the reference IC. The drift due to aging typically halves with each doubling of time, so each reference is aged for at least 200hours at it’s calibration temperature and supply voltage.
Final calibration and adjustment happens after each reference has been aged, the results are recorded and the calibration label applied to the back of the reference. We use a UKAS calibrated Keysight 34465A for the calibration and adjustment of both the 389-1 and 389-2 references.
We haven’t had an incidence of ‘wheeltappers’ recently!