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4 Measurement Tutorials
Noise caused by injected current
Residual capacitance in the instrument's power transformer cause small currents to flow from the LO terminal of the
internal DMM to earth ground. The frequency of the "injected current" is the power line frequency or possibly
harmonics of the power line frequency. The injected current is dependent upon the power line configuration and
frequency. A simplified circuit is shown below:
With Connection A (see below), the injected current flows from the earth connection provided by the circuit to the
LO terminal of the internal DMM. This configuration adds no noise to the measurement. However, with Connection
B, the injected current flows through the resistor R thus adding noise to the measurement. With Connection B,
larger values of R will make the problem worse.
The measurement noise caused by injected current can be significantly reduced by setting the integration time of
the internal DMM to 1 PLC or greater.
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Keysight DAQ970A/DAQ973A User's Guide
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