Keithley 6485 Instruction Manual page 232

C-6 General Measurement Considerations Model 6485 Picoammeter Instruction Manual
The effect on instrument performance can be considerable if enough of the unwanted sig-
nal is present. The effects of EMI can be seen as an unusually large offset, or, in the case of
impulse sources, erratic variations in the displayed reading.
The instrument and experiment should be kept as far away as possible from any EMI
sources. Additional shielding of the instrument, experiment and test leads will often
reduce EMI to an acceptable level. In extreme cases, a specially constructed screen room
may be required to sufficiently attenuate the troublesome signal.
External filtering of the input signal path may be required. In some cases, a simple one-
pole filter may be sufficient. In more difficult situations, multiple notch or band-stop fil-
ters, tuned to the offending frequency range, may be required. Connecting multiple capac-
itors of widely different values in parallel will maintain a low impedance across a wide
frequency range. Keep in mind, however, that such filtering may have detrimental effects
(such as increased response time) on the measurement.