Digital Multimeters: 2000, 2001, 2002, 2010, 2700, 2701, 2750
Question: What is pumpout current and when might it pose a problem for my measurements?
Answer: Pumpout current is a small discharge of current at the meter input. This current discharge is a result of the switching the instrument performs between measurements of the input signal and the background measurements performed by the analog to digital (A/D) converters, which include measurement of reference and zero points.
Pumpout current can sometimes be a problem for low-level signals. Measurements made by DMMs and nanovoltmeters in particular, can be affected by pumpout current. A symptom of a pumpout current problem is when there appear to be noise injections in the measurement results. It may be easier to consider pumpout current as being bursts of charge. These small charges develop an error voltage across the source of voltage being measured.
To reduce the effect of pumpout current, you may do one of two things: (1) Disable autozero, which will remove A/D switching from the input signal to the reference and zero points. However, disabling autozero for extended periods of time will cause measurements to drift. You will want to periodically re-enable autozero, every 10 minutes or so, to update the background measurements. Some instruments have an AutoZero once command, which forces an immediate update of the zero and reference readings. (2) Choose a meter with a low pumpout current. The Keithley Model 2182 Nanovoltmeter has the lowest pumpout current of any nanovoltmeter in the industry.