Agilent Technologies 34970A, User Manual, Page: 291 / 356

291
Chapter 7 Tutorial
Measurement Fundamentals
4
7
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 .
I
test
HI
LO
To Amplifier and
Analog-to-Digital
Converter
R
unknown
I
34970A Refresh UG.book Page 291 Wednesday, February 17, 2010 12:34 PM