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.
The noise caused by injected current can be significantly reduced by
setting the integration time of the meter (NPLC) to 1 or greater.
See page 252.
Injected
Current
Injected
Current
CONNECTION A
CONNECTION B
Ideal
Meter
Ideal
Meter
Ideal
Meter
Injected
Current
7
Chapter 7 Measurement Tutorial
Measurement Techniques and Sources of Error
257
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