Agilent Technologies 34970A manual Resistance Measurements

Chapter 8 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.

HI

Runknown Itest

To Amplifier and

Analog-to-Digital

Converter

I

LO

The internal DMM offers two methods for measuring resistance:

2-wireand 4-wireohms. 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.

8

369