Hi Dave, Fundamentally, any input impedance from your 3458 is going to throw your 720A out of wack. the 720A is not supposed to have even picoamps of loading.... nothing, nada, zip.
I would suggest that before you damage your 720A further, you read the manual for the 720A and follow the instructions to the letter. An HP3458A is not a substitute for a null detector and a voltage calibrator. The calibration of a 720A requires that no current, none, zip, be drawn from the 720A on any port. It can only be done by using a voltage calibrator, and a null detector, as a differential voltmeter. Remember, and heed my words, *no* current may be drawn from the 720A. -Chuck Harris David C. Partridge wrote: > Update: After changing the resistor I added in series with R1008 (A decade > position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means that R302 is > slowly > increasing in value back towards nominal) and re-calibrating again, the A > decade > is now within 1ppm of linear (using the 10V range and nominally 10V input). > > I note that when I'd done this, with the A decade set to 0.9 (reading > 9.000000V on > the 3458A), the actual input voltage needed was 10.000040 or so. Is that to > be > expected? > > Two of the positions on the B decade wouldn't quite adjust for a null when > calibrating it. In this situation I think I have two options: > > 1) Add series resistors to compensate for slightly low value 9.898k resistors > in > the relevant two positions of the decade and recalibrate. > > 2) Adjust R203 to change the bridge balance slightly to the 10K ohm decades > and > recalibrate. > > Your thoughts on which approach to take is much appreciated. > > Thanks Dave > > _______________________________________________ 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. > _______________________________________________ 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.
