TiN did quite some tests and tries to fix it with his 1st bad ADC board. The conclusion was that the resistors in the ASCIC somehow started to dirft a bit. Not all resistors are acessible externally and replacing the ASIC completely can be tricky - there are switches with a difined ratio in the switch resistance. It is a bit more complicated than just multiple equal resistance switches. More like having 1:4:5:16 ratio in the switch resistance.
Yep, they are a bit of mystery. I have an A3 that was drifting at 0.4 to 0.8 ppm/day. After a bit of a "fix" it is now ~ 0.02 ppm/day. Could be a total fluke, maybe it won't last. I just want another board to try the process on.
I have a 3458A that is drifting quite badly: averaging 2ppm per day, totalling 16ppm drift over 8 days running from a cold start. The indications are that the drift rate is even accelerating as time goes on.
Would you be prepared to publish your fix? I don't have a spare board so I can't ship the faulty one to you.
I might be interested to try your fix if I fail to get any compensation from the the vendor who described the 3458A as being in good condition in the auction listing.