Hi Mickle,
I have been looking at the documentation and circuits of the 7071/7081 and I
think you have overlooked something.
According to the manual, if nines=6 or greater then drift=off does not do
anything.
Also looking at the large tempco that you had I suspect that the reference
zener had drifted enough that the minimum drift current setting was now
incorrect for your unit, you should have been able to correct the drift by
tinkering with the current setting number, and not by adding copper at +3700ppm
c-1. OR the current delivering circuit had changed enough that the zener
current was wrong. They are designed to have a minimum tempco at approximately
7.5mA, but for any individual item it may be different from that current.
I used to be a service agent for Dranetz and their 305c's used a reference that
was good for 6 to7 digit stability using the in829's and the required current
could vary quite a bit. There was no particular use of any high stability
resistors in the reference they designed, but it worked quite well.
Regards,
M K
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