[volt-nuts] Drifting 3458As

[Randy Evans]

I have been testing two 3458As against a known good Fluke 732A. Each 3458A
was calibrated for DCV before the start of the measurements so they both
started at the same point.   I have been running a series of tests
consisting of measuring the Cal Constant as detailed in Service Note 18 for
each meter several times a day (to calculate the drift per the procedure in
SN 18).  I also measure the 732A voltage in each meter each time using NLPC
100 and NRDGS 100 and then recording the STDEV, MEAN, MAX, and MIN values.
What I have observed is that the Cal Constant is acceptably low but the
MEAN value per measurement  is drifting up in one meter and drifting down
in the other.   The unit drifting down has a new A3 board installed and the
unit drifting upward is an Agilent 3458A only a few years old so would not
be expected to have a drifting A3 board.  It was only calibrated 1 time per
the internal REV number, so would have likely been re-calibrated if the A3
board was replaced.


 My question is what is the likely cause of the drift in the MEAN voltage
reading if the Cal Constant value is relatively constant?  SN 18 says the
drift rate of the Cal Constant is an indication of drift in the A3 AD
board, but I believe it assumes the A9 voltage ref board has a constant
value over time.  If this is not true (e;g., the voltage ref is drifting),
I think this would explain the drift in the voltage reading even though the
Cal Constant is relatively constant.  Any opinions on this?


Thanks,


Randy Evans