Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 137)

Section 7. Measurement Instructions
7-11
7.5 Current Excitation
Resistance (Dest, Reps, Range, DiffChan, IxChan, MeasPEx, ExuA, RevEx,
RevDiff, SettlingTime, Integ, Mult,Offset)
The Resistance instruction applies a precision current excitation and measures
a differential voltage. The result is the measured voltage divided by the
excitation current. The measurement can be used to measure resistance or, with
some sensors, to provide a current excitation for measuring a full bridge.
The excitation current is set in the instruction with a maximum current of 2500
microamps. The compliance voltage is
± 5 volts. The compliance voltage is
the maximum or minimum voltage to which the excitation can go in order to
drive the current specified. Some of the consequences of this are:
The maximum resistance that can be excited with 2500 µA= 5V/0.0025A =
2000 Ohms.
The maximum resistance that can be excited with 1000 µA = 5V/0.001A =
5000 Ohms.
The minimum resistance required to fill the 20 mV range = 0.02V/0.0025A = 8
Ohms.
The resistance instruction can be used for full bridge measurements were the
sensor is calibrated for current excitation or the resistance across the bridge is
constant (or assumed to be). The 2.5 mA maximum current excitation
provides for reasonable output from the full bridge when the bridge resistance
is in the range of 2000 to 5000 ohms. For lower resistance bridges such as 120
ohm or 350 ohm, measuring with the voltage excitation full bridge BrFull will
allow a higher output from the bridge.
Resistance used to measure resistance
H
L
H
L
I
IXR
X = result w/mult = 1,
offset = 0
X
V
I
R
x
s
= =