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Chapter 2
Connecting Signals
©
National Instruments Corporation
2-5
NI PXI-4220 User Manual
Figure 2-4.
Quarter-Bridge I Circuit Diagram
The following symbols apply to the circuit diagram and equations:
•
R
1
and R
2
are half-bridge completion resistors.
•
R
3
is the quarter-bridge completion resistor (dummy resistor).
•
R
4
is the active element measuring tensile strain (+
ε
).
•
V
EX
is the excitation voltage.
•
R
L
is the lead resistance.
•
V
CH
is the measured voltage.
•
R
S
A
is shunt calibration resistor A.
•
R
S
B
is shunt calibration resistor B.
This configuration uses the Shunt Cal A switch for shunt calibration.
Note
The value of the quarter-bridge completion resistor (dummy resistor) must equal the
nominal resistance of the strain gauge. NI recommends using a 0.1% precision resistor.
If a less precise resistor is used, offset null compensation calibration is not as accurate.
Refer to the
Offset Null Compensation
section of Chapter 4,
Theory of Operation
, for more
information.
+
–
+
–
R
L
R
L
NI PXI-4220
Transducer
R
1
R
2
R
4
R
3
V
CH
S+ (Pin 1)
P+ (Pin 2)
P– (Pin 7)
Shunt Cal B
R
L
SCCOM (Pin 5)
QTR/SCB (Pin 9)
V
EX
R
S
B
P+
QTR
P–
S–
Shunt Cal A
R
S
A
SCA (Pin 4)
Jumper not required
when using SCB.
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