7Measurement Tutorial
282 Keysight 34420A User’s Guide
Noise Caused by Injected Current
Residual capacitances in the power transformer cause small currents to flow from
the LO of the meter to earth ground. The frequency of the “injected current” is the
power line frequency or possibly power line frequency harmonics. The injected
current is dependent upon the power line configuration and frequency. You will
need to recalibrate the injected current if you operate the meter with a different
power line than the power line used to calibrate the meter. A simplified circuit is
shown.
In the circuit below, with connection A the injected current flows from the earth
connection provided by the circuit, to the LO terminal of the voltmeter, and no
noise is added to the measurement. However with connection B, the injected
current flows through the resistor R adding noise to the measurement. With
connection B, larger values of R will make the problem worse.
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