Measurement Computing CIO-EXP-GP - Manual (Page 38)

A Half Bridge Example
For a ½ bridge circuit (Figure 7-7), the strain gauge has two resistive elements which are connected
across two legs of the bridge. The two legs would always be A & C or B & D. The other two legs of the
bridge must be populated with the precision (350 ohm) completion resistors.
GAIN SW
AMP
EXCITATION VOLAGE (+)
TO CHANNEL
MULTIPLEXOR
EXCITATION. VOLTS (+)
SENSE LOW (-)
EXCITATION VOLTS (-)
SENSE HIGH (+)
80 Hz Low
Pass Filter
A
B
C
D
Arm
Null Pot
Figure 7-7. ½ Bridge Circuit- Simplified Schematic
Half Bridge Calculations
The ½ bridge implementation consists of two strain gauges; one on the top of the beam (as in the ¼
bridge example) and one on the bottom of the beam. The strain gauge on the bottom of the beam
replaces completion resistor C in the ¼ bridge implementation.
Two active strain gauge elements (one in tension and one in compression) result in twice the sensitivity
of the ¼ bridge. (One element increases resistance while the second element decreases resistance
simultaneously.)
When the beam is forced down (250µ
ε
change), the resistance in C decreases by 0.175 ohm, and
resistance A increases by 0.175 ohm as shown in the ¼ bridge example above.
The bridge voltage V
br
is then:
V
br
= 10V { (350 / 700) - [(350 - 0.175) / ( (350 - 0.175) + (350 + 0.175) ) ] }
V
br
= 10V {(350 / 700) - [(349.825) / (700) ] }
V
br
= 2.500mV
Choosing Gain = X1000 would result in 2.5V being applied to the DAS board. Choosing Gain = X2500
(X1000 on the input channel and X2.5 on the output) could result in an amplified voltage that’s out of the
DAS board’s range. In this case, the excitation voltage could be reduced to 4V, reducing the bridge
voltage to 1.00mV. A gain selection of 2500 would then present a maximum voltage of 2.5V to the DAS
board.
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