[volt-nuts] Solartron 7150Plus and 7151 zero offset?

[David C. Partridge]

I'm trying to understand what is causing the DC zero offset I'm seeing in these meters (one is showing around 30uV).  Both use the same floating board (apart from the ROM content).

I have checked and adjusted the offset on IC1a (OP14) for a zero reading, and the reference adjustment for the forcing voltages is set for both plus and minus 3V to be the same value.  In fact misadjustment of the bias on IC1a is only observable for a short while as the ten second drift correction cancels it out.

I don't think the problem is an offset in IC8 (OP05) at the input to the ADC, as any offset in that should be cancelled by the forcing square wave.

The drift correction works by turning off FET TR5 (in series with input), and turning on FET TR6 to short the input of IC1a to signal 0V.  The logic being that any observed count with zero input must be due to drift.  The observed count is then subtracted from subsequent readings.

The intent is to fix the problem, but I can't do that until I understand why it is happening.

Thanks
Dave