Measurement Computing CIO-EXP-GP - Manual (Page 37)

A Quarter Bridge Example
For ¼ bridge circuits, the strain gauge has a single resistive element that is connected as one leg of the
bridge. The other three legs must be populated with the precision completion resistors.
GAIN SW
AMP
EXCITATION VOLAGE (+)
TO CHANNEL
MULTIPLEXOR
EXCITATION. VOLTS (+)
SENSE LOW (-)
EXCITATION VOLTS (-)
SENSE HIGH (+)
80Hz Low
Pass Filter
A
B
C D
Arm
Null Pot
Figure 7-6. ¼ Bridge Circuit - Simplified
Quarter-Bridge Calculations
The strain gauge is applied to the top of the beam. This strain gauge takes the place of resistor A
(see Figure 7-6). Three other 350 ohm resistors (B, C and D)
complete the bridge circuit. These are
installed by the user in locations provided on the board or attached to the screw terminals.
As downward force is applied, the strain gauge on the top of the beam will be stretched, therefore
its resistance will increase by:
Strain Gauge increase = 350 ohm x 250 x 10 -6 x 2
= +0.175 ohm
Thus the value of gauge A (under tension) will be 350.175 ohms when the strain on the beam is +250µ
ε
.
Initially, choosing an excitation voltage of 10V, the bridge voltage is:
V
br
= 10V { (350 / 700) - [350 / (350 + 350 )) ] } = 0V
After a downward force is applied:
V
br
= 10V { (350 / 700) - [350 / (350 + 350.175)] }
V
br
= 1.25mV
Choosing an amplifier gain of X1000 results in 1.25V maximum presented to the DAS board.
Choosing an additional X2.5 (overall output gain) results in a total gain of 2500, thus sending
3.125V maximum to the DAS board. This makes an optimum use of the 5V range.
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