Why do I measure an offset current from an unbiased photomultiplier tube (PMT)?

Why do I measure an offset current from an unbiased photomultiplier tube (PMT)?

All of Keithley's newer picoammeters and electrometers use a feedback ammeter configuration. For this type of ammeter to work properly, the source resistance of the device-under-test must be greater than the feedback resistance of the ammeter. As the source resistance decreases below the value of the feedback resistance, the voltage gain of the amplifier increases significantly and any input voltage offset or noise will be amplified and appear as a large offset current on the meter display. PMTs generally have a very high source resistance that makes a picoammeter or electrometer the ideal measurement instrument to use with them. However, for observing higher speed phenomena with an oscilloscope or other instrument, some PMT designs include a resistor (commonly 50 ohms) across the output to increase its frequency response. If you connect a PMT with such an output resistor to a feedback ammeter, then you will likely see the large offset you observed.

Check your PMT documentation to determine if it has an output resistor, or use an ohmmeter or DMM to measure it yourself. You can sometimes remove this resistor, which will enable you use your picoammeter or electrometer without the apparent offset. On the other hand, if you leave the resistor in place, it effectively converts the current signal to a voltage signal that you can measure with a DMM or other suitable instrument.

For more information about electrometers, picoammeters and photomultiplier tube measurements, refer to Keithley's Low Level Measurements Handbook.