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3458A DMM timing error during DCV sampling

Question asked by amia on Oct 5, 2010
Latest reply on Oct 14, 2010 by amia
Hi all!

I am using 3458A with option 001 in DCV sampling mode to measure properties of sinus signals from hundereds up to 8000 samples. I have observed, that DMM makes sometimes delay 100 ns in sampling.

The question is: What can be the reason for the 100 ns delay and how to prevent from it?

During exploring this problem, I simplified measurement setup up to the following combination, to ommit possible interferences from other sources. (In original setup, DMM has added home made small circuit to provide optical clock signal for synchornization of other devices). So he is simple test setup:
  1) DMM input is connected to the 33220A output.
  2) DMM is programmed via HPIB-USB adaptor for TRIG SGL
  3) HPIB is disconnected.
  4) DMM is triggered via ext.trigger
  5) ext.trigger cable is disconnected
  6) when sampling is finished, HPIB is connected and samples read out from the DMM to the computer.
I repeated this experiment outside of measurement laboratory in the other room, on the other floor, with different DMM. The problem still exists.

The problem with 100 ns delay occurs sometimes less, sometimes very often. Up to now, I have tried and found following things:

- the problem is not due to some delays in generated signal. (I have sampled with DMM signals from different generators , moreover I have sampled same signal in parallel with PXI high   speed digitizer. Generated signal is really clear)
- it is ussual, that the problem occurs one day at some combinations of aperture time, other day or week on other combinations
- this problem was observed by other users too, but only after hundreds or thousands of samples. The idea for this problem was, that  DMM has problem with timing during saving data to memory. Now, I see the problem sometimes during first hundred of samples too.
- sometimes it seems, that 100ns delay occurs at same sample nummer, sometimes this delay "floats" along samples and change position in sampled signal.
- when I place some EMI filter before the 3458A power plug, the problem dissapears for some DCV mode aperture time configurations, but newly appears at different configurations. Same happens sometimes, when I change the capacitance (e.g. I use longer cable for trigger input) on the trigger input of the DMM.
- this problem occurs not only with one DMM, I have seen this problem on 3 DMM, two of the quite new, one was older.
- details mentioned above says me, that there is some problem in the power net but I am not sure, because I have access to more power nets in the laboratory and changing them has no effect.

Sorry for long text and thanks in advance for response