Electronics > Metrology

Anyone have more than one 3457a and compared their DC readings?

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Here is the bias data for the two meters. There was quite a time difference between the two sets of data collected from meter 2 and the reading had shifted in that time.

Meter 1 R=9.59Mohm
Meter 2 R=9.71Mohm
Humidity changed from 52% to 56% as the electrical storm passed by.

Looks good.

Here is the data from the re run of the noise test.

Just a bit of an update on progress with the 3457a.

I've tried a few different jfets and changed a couple of other components in the input amplifier section to arrive at what seems to be a lower noise level.
The bias on meter 2 has gone up in the process. M1 currently reads 19µV whilst M2 reads 62µV.

The bias current comes from semiconductor leakage, mainly the FETs and possibly some protective devices, but also from just leakage on the board. Leakage on the board can go up with contamination but also from just humidity / alcohol that diffused inside. So directly after cleaning it might be higher and it might need a few days to come down.

The noise looks good, or as good as one can get from this design - quite some noise is just from the input resistors used for protection, and also the ADC itself it not that low in noise, as you can see in the 3 V range.


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