Electronics > Metrology

HP 34401A hacks and upgrades

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The linked other thread has indeed a really great describtion of the software details.
A somewaht tricky point with measuring the charge on the fly is the right timing on when the ADC is sampled. Dending on the exact time when the µC internal ADC is sampling a fraction of the last run-up count (pos or neg reference) may be included or not.  For the software description it looks like they adjusted the timing just right to have only full cycles included and no frational part.

Another interesting point is that they include a numerical correction for an U² part of the ADC nonlinearity. This part would to a large part come from the 74HC4053. So changing that chip (and possibly also just the supply of this chip)  would requite a new (may be special low level ?) calibration. They could get this correction parameter from the cal steps with +10 V and -10 V so it could be part of the normal calibration.

I am a bit surprized to also see a correction for a U³ part. AFAIK the normal calibration does not include an extra test point for 5 or 7 V to also measure this parameter. So to measure this correction may need a special (low level) calibration.

im not sure about AC linearity , as it basically AC-DC RMS AD637 converter , same i see in the fluke 8840\42 ;  same in Keithley 175 (4.5)  197(5.5)  ,  and many others .


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