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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)

Connection A

Connection B

DMM

Chapter 8 Tutorial

Measurement Fundamentals

356

HP 34970A User Manual

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