HP 34970A Noise Caused by Injected Current

Chapter 8 Tutorial

Measurement Fundamentals

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.

HI

DMM

LO

Injected Current

(50 / 60 Hz ac line leakage current)

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.

	R				R
VS		HI		VS		HI
			DMM			LO	DMM
		LO

Connection A Connection B

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).

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