User`s guide

289

Chapter 7 Tutorial

Measurement Fundamentals

Current Measurements

Current measurements are allowed only on the 34901A module.

An ammeter senses the current flowing through its input connections –

approximating a short circuit between its input terminals. An ammeter

must be connected in series with the circuit or device being measured

such that current flows through both the meter and the test circuit.

A resistor, R

in the diagram below, is connected across the input

terminals such that a voltage drop proportional to the input current is

generated. The value of R

is selected as low as possible to minimize the

instrument’s burden voltage, or IR drop. This voltage drop is sensed by

the internal DMM and scaled to the proper current value to complete the

measurement (see the discussion on the following page).

AC current measurements are very similar to DC current

measurements. The output of the current-to-voltage sensor is measured

by an AC voltmeter. The input terminals are direct coupled (ac+dc

coupled) to the shunt so that the internal DMM maintains DC continuity

in the test circuit. Performing AC current measurements demands

additional care. The burden voltage (loading) varies with frequency and

input inductance, often causing unexpected behavior in the test circuit

(see the discussion on the following page).

On-Card

Fuse

±VDC to Input Amplifier and

Analog-to-Digital Converter

34970A Refresh UG.book Page 289 Wednesday, February 17, 2010 12:34 PM