[volt-nuts] Agilent 3458A Issues

[Randy Evans]

I just received an Agilent 3458A that has a problem with noise and a
drifting voltage measurements.  I am using two Fluke 732As to compare
absolute voltage measurements over time against the Agilent and an HP
3458A.  The HP unit has a new A3 ADC card and seems to be very stable and
low noise, so is being used for comparison.  I have been doing simultaneous
absolute voltage measurements and DC Cal Constant measurements several
times a day and then calculating the drift rates of the two units using the
HP Service Note 18 procedure.



The results indicate the Cal Constant drift rate of both units is very
similar and within spec per Service Note 18.  However, the absolute value
measurements show the Agilent unit changing 1.1 ppm over a day whereas the
HP unit is within a tenth of a ppm over a day.  In my way of thinking the
Cal Constant procedure assumes the voltage reference board in the 3458A is
stable, hence the absolute value reading should remain essentially constant
after each ACAL DCV, which is the case with the HP unit.  Since the Agilent
unit shows a steady drift in the absolute reading, this would indicate to
me that the voltage reference board is likely the cause of the problem, and
is also likely the cause of the noisy readings.  If so, this is a
“relatively” easy fix (I have several 3458A voltage reference boards, one
of which has been continuously powered up for several years).



The issue is that I have to make a decision to keep or return the Agilent.
It has a cal seal on it and if I open the unit up to change the voltage
reference board, I own it and can’t return it.  I would appreciate an
opinion from the members of the group as to what they think the odds are
that the voltage reference board is the source of the problems with the
Agilent 3458A.



Thanks,



Randy Evans
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