Keysight 34972A - Page 291

Tutorial 7
Keysight 34970A/34972A User’s Guide 291
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
133 for a discussion of integration time).
DMM
Injected Current
(50/60 Hz ac line leakage current)
HI
LO
DMM
DMM
HI
HI
LO
LO
V
S
V
S
R
R
Connection A Connection B