HP 34970A manual Connection a Connection B

Chapter 8 Tutorial

Measurement Fundamentals

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instrument’s power transformer cause small currents to flow from the VY terminal of the internal MWW 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 T” R (see below), the injected current flows from the earth connection provided by the circuit to the VY terminal of the internal MWW. This configuration adds no noise to the measurement. However,

with T” S , 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				DMM

LO

LO

Connection A Connection B

The measurement noise caused by injected current can be significantly reduced by setting the integration time of the internal MWW to 1 ZVL

or greater (z z ” z ).

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