22
Chapter 4
.
M
EASUREMENTS
M
ODES AND
F
UNCTIONS
The main characteristic of the 4176 is 4-wire resistance measurements. In addition to
standard measurements, the 4176 presents the TCM and HLC modes. These features
may significantly ease time and cost of temperature sensitive and automated selection
applications.
4.1 Standard Measurement Mode
The 4176 powers on in the standard measurement mode, in this mode, the instrument
simply measure the value of the applied load using a 4-wire configuration. This
configuration eliminates errors
normally caused by test lead and
contact resistances. In many
applications
the
contact
resistance can exceed the value
of the load by several orders of
magnitude. The 4176 bypasses
this potential error source by
providing two terminals of
constant
current
and
an
additional two terminals for
high
impedance
voltage
measurement. The result is a
fast,
accurate
resistance
measurement of the load,
independent of the resistance of
the current carrying leads.
The figure on the right illustrates how the 4-wire principle is used to eliminate lead, wire,
and contact resistances as potential error sources. The internal current source inherently
overcomes all series resistance (within compliance voltage limits) and delivers a precise
constant current. The internal high-impedance DVM senses the voltage drop across the
load. There is negligible contact and lead resistance error created by the voltage
measurement because the high input impedance of the DVM limits current flow in the
voltage leads.
