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
HP-3458A Calibration drift rate 2017-11-17A.xlsx
Description: MS-Excel 2007 spreadsheet
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