Noise Caused by Injected Current
Residual capacitance in the instrument’s power transformer cause small currents to flow from the LO ter-
minal of the internal DMM to earth ground. The frequency of the “injected current” is the power line fre-
quency 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 cir-
cuit 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.
Loading Errors Due to Input Resistance
Measurement loading errors occur when the resistance of the device-under-test (DUT) is an appreciable
percentage of the instrument’s own input resistance. The diagram below shows this error source:
4Measurement Tutorials
196 Keysight DAQ970A User's Guide
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