Campbell 21X Operator's Manual page 153

SECTION
13.
21X MEASUREMENTS
*Vx
3
Wire
Half Bridge
Differential
Measurement
with Excitation
6
Wire
Full Bridge
or 4
Wire
Half Bridge
Compensates
for lead
wire resistance,
assuming resistance is
same in both wires.
Two single-ended
measurements at each
excitation polarity.
Ratiometric output.
Makes a ditferential
measurement
without
reversing excitation
polarity
or switching
inputs.
Used for fast
measurements
on load
cells, PRTs etc.
Resolution and common
mode rejection worse
than 6 if used with
delay=g.
Measured
voltage output.
Compensates
for lead
wire resistance. Two
differential
measurements at each
excitation polarity.
Ratiometric output.
I
123456
Time
(milliseconds)
FIGURE
13.5-2. Excitation and Measurement
Sequence
for
4
Wire Full Bridge
TABLE
f
3.5-1. Comparison of Bridge
MOasurement
Instructions
lnstr.
CircUit
Description
AC
Flalf Bridge
4 Wife
Full Eridge
Calculating
the actual resistance of a sensor
which
is
one
of
the legs of a resistive bridge
usually requires
the use of one or two
Processing Instructions
in
addition to the bridge
measurement
instruction. lnstruction
59 takes
a
value,
X,
in
a specified input location and
computes the
value M)U(1-X), where M
is
the
multiplier and stores
the result
in
the original
location.
lnstruction 42 computes
the reciprocal
of a
value in an input location. Table 13.5-2
lists
the instructions used to compute the
resistance of any single resistor shown in
the
diagrams in Figure 13.5-1, provided the values
of
the other resistors
in
the bridge circuit are
known.
DC Flalf
Bridge
User entered settling
time allows
compensation
for
capacitance in long lead
lengths. No polarity
reversal.
One single-
ended measurement.
Measured voltage
output.
Rapid
reversalof
excitation polarity
for ion
depolarization. One
single-ended
measurement at each
excitation polarity.
Ratiometric output.
Slightly lower noise than
9.
One differential
measurement at each
excitation polarity.
Ratiometric output.
13-17