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6031A Common Mode Calibration

Question asked by wonghoi on Nov 13, 2011
Latest reply on Nov 30, 2011 by PeterW
I'm trying to perform the Common Mode Calibration procedures set forth in page 15 of the 6031A service manual ( ... 9-3344.pdf) and have a few questions:

1) Section b) says short power supply sense terminals (+S to -S) at rear panel, but if the local sensing straps are connected, does it also mean shorting the output terminals as well?
2) I did the setup in Figure 2-1 but the display says OL (overload) for voltages. Did I do anything wrong? I checked to polarities and removed the local sense strap.
3) If I performed Voltage Monitor Zero Calibration, should the initial reading in Common Mode Calibration be close to zero, as achived by the Voltage Monitor Zero Calibration?
4) I tried either way for Common Mode Calibration. Initial readings are either 0.065mV (local sense strap disconnected) or 0.00001mV (with local sense strap connected to output), but after apply 1V between -S (- sense) and -V (- output), I get something like -250mV and adjusting A2R21 all the way only give me somewhere -247mV to -252mV, nowhere close to the initial reading. Am I doing something wrong?

I'm using HP 3478A multimeter and E3640A to generate 1V, and yes, I had a 1kOhm resistor to measure V-MON.

Any advice is appreciated. Thanks a lot in advance.