HP 34401A, User Manual

Page: 202 / 236

Resistance Measurements
The HP 34401A offers two methods for measuring resistance: 2-wire
and 4-wire ohms. For both methods, the test current flows from the
input HI terminal and then through the resistor being measured. For 2-wire
ohms, the voltage drop across the resistor being measured is sensed
internal to the multimeter. 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.
The errors mentioned earlier in this chapter for dc voltage measurements
also apply to resistance measurements. Additional error sources unique
to resistance measurements are discussed on the following pages.
4-Wire Ohms Measurements
The 4-wire ohms method provides the most accurate way to measure
small resistances. Test lead resistances and contact resistances are
automatically reduced using this method. Four-wire ohms is often used
in automated test applications where long cable lengths, numerous
connections, or switches exist between the multimeter and the device-
under-test. The recommended connections for 4-wire ohms
measurements are shown below.
See also “To Measure Resistance,”
on page 17.
I
test
Ideal
Meter
HI
LO
LO-
Sense
HI-Sense
R
=
V
meter
I
test
7
Chapter 7 Measurement Tutorial
Resistance Measurements
203