User`s guide

291

Chapter 7 Tutorial

Measurement Fundamentals

Resistance Measurements

An ohmmeter measures the DC resistance of a device or circuit

connected to its input. Resistance measurements are performed by

supplying a known DC current to an unknown resistance and measuring

the DC voltage drop.

The internal DMM offers two methods for measuring resistance: 2-wire

and 4-wire ohms. For both methods, the test current flows from the input

HI terminal through the resistor being measured. For 2-wire ohms, the

voltage drop across the resistor being measured is sensed internal to the

DMM. Therefore, test lead resistance is also measured. For 4-wire ohms,

separate “sense” connections are required. Since no current flows in the

sense leads, the resistance in these leads does not give a measurement

error.

4-Wire Ohms Measurements The 4-wire ohms method provides the

most accurate way to measure small resistances. Test lead, multiplexer,

and contact resistances are automatically reduced using this method.

The 4-wire ohms method is often used in automated test applications

where long cable lengths, input connections, and a multiplexer exist

between the internal DMM and the device-under-test.

The recommended connections for 4-wire ohms measurements are shown

in the diagram on the following page. A constant current source, forcing

current I through unknown resistance R, develops a voltage measured by

a DC voltage front end. The unknown resistance is then calculated using

Ohm’s Law.

test

To Amplifier and

Analog-to-Digital

Converter

unknown

34970A Refresh UG.book Page 291 Wednesday, February 17, 2010 12:34 PM