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Noise Caused by Injected Current Residual capacitances 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 (see page 103 for a discussion of integration time).
DMM
Injected Current
(50 / 60 Hz ac line leakage current)
HI
LO
Connection A
Connection B
DMM
DMM
V
S
R
HI
LO
V
S
R
HI
LO
Chapter 8 Tutorial
Measurement Fundamentals
356