Keithley DMM - Canceling Input bias

Keithley could probably answer that, but I'd be very suprised if that adjustment was done anywhere very near the front end.

Best regards, Spehro Pefhany

Connect it to a capacitor, let it measure its own input current, and try it!

Does it use a mechanical chopper?

John

John,

Thanks for the help. I'll see what voltage it measures across a discharged cap. I'm sure it will charge the cap far beyond its 1uV resolution, correct? I believe the 160B has a solid state chopper, but I believe the older models such as the 150B have mechanical choppers. The 160B manual says,

"A typical Model 160B has an internal offset of about -100uVolts in the chopper amplifier. Hence, adjusting the Front Panel Zero from end to end will typically produce about +125uVolts and -325uVolts. This "extra" adjustment at the negative end of the range allows sizeable positive offsets to be compensated for, as is the case when the Zero is used for test lead compensation on the low-ohms Ranges. 200m ohms of test lead resistance will produce +200uVolts of zero offset, which can be "bucked out" by the Zero control. (See also Section 3-5d). Zeroing of the chopper amplifier output is actually accomplished by having the "Zero" circuit either source or sink whatever current is present in feedback resistor R160D. For example, an Offset voltage of

-111uVolts on the feedback line produces a current of -1uA in resiseor R160D (111.135 ohms). A voltage of -1uVolt at the wiper of Zero potentiometer R114 will produce an equal current of -1uA in resistor R115 (1M ohm). With this zeroing scheme, for the chopper amplifier output to be at zero, the voltage at the feedback line will always be offset from the voltage at the input by a constant, say for example. -111uVolts. (The feedback line connects to the junction of R160C and R160D)."

I was wondering, what about using the Keithley to measure current? I don't see any reference to bias current in the manual for measuring current.

Paul