Illya, Can you explain why you say " If ACAL DCV does not remove drift then A3 is probably fine". I don't really follow the argument.
Randy On Thu, Nov 23, 2017 at 5:18 PM, Illya Tsemenko <il...@xdevs.com> wrote: > Well, one thing you can know for sure , that reference is indeed broken. > Drift over a day should be way below the noise floor (<0.1ppm). With 1.7x > gain of ADC scale that drift rate gives you around 0.8ppm +/- 0.3ppm due to > zener noise. So it is in line of 1.1ppm/day. If ACAL DCV does not remove > drift then A3 is probably fine. > > Since reference is much easier to troubleshoot and fix I would go with > replacing LTZ chip and testing if drift go away, if that have any help on > your desire to keep meter. > > Also serial number range in SN doesn't mean much for you, as meter history > is unknown and it still may have been serviced at some time. > > On November 24, 2017 8:40:00 AM GMT+08:00, Randy Evans < > randyevans2...@gmail.com> wrote: >> >> Illya, >> >> I ran the test you suggested over 4 days and got 7.19114068 VDC on the >> start of the test and 7.19113736 VDC at the end of the fourth day (96 hours >> later). I calculate a total of 0.46 ppm drift, which seems excessive but >> does not account for the 1.1 ppm/day I measured overall. I suspect the A3 >> card is the primary source of drifting. Since the unit is a late model >> Agilent unit, that is well beyond the expected range of units described in >> Service Note 18. What do you think? >> >> Thanks, >> >> Randy Evans >> >> On Sat, Nov 18, 2017 at 4:47 AM, Illya Tsemenko <il...@xdevs.com> wrote: >> >>> Since you have 732A, testing should be easy enough. Calibrate faulty >>> meter for zero and DCV 10V to 732A, record CAL? 2,1 value. This is your LTZ >>> output. Then leave it running for few days to drift away and calibrate >>> again to same 732A. Check CAL? 2,1 again. Calculate the difference and if >>> it matches output drift (that 1.1ppm/day you mention) - you can be 80% sure >>> that A9 is a problem. Other 19% go to A1 and A3 circuits, as 7V is not used >>> directly in the meter, and there are still gain parts to get +12 and >>> -12VREF on A3 and 10Vish bipolar levels on A1. If your CAL? 2,1 stays same >>> (within 0.3ppm) then A9 is fine. >>> >>> >>> On November 18, 2017 12:59:53 PM GMT+08:00, Randy Evans < >>> randyevans2...@gmail.com> wrote: >>>> >>>> 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 >>>> >>>> >> _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.