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© National Instruments Corp. 13 NI 9235/9236
Quarter-bridge measurements are inherently sensitive to accuracy
degradation due to the lead resistance of wiring from the sensor to
the measurement device. For a given change in the gage resistance,
the total effective resistance changes slightly less. Accordingly, the
measured mV/V reading is less than its true value. However, you
can use shunt calibration to quantify the lead wire desensitization,
and can then design the software application to correct subsequent
readings for this gain error. The gain error caused by a lead wire
equals R
L
/R
G
, where R
L
is the lead wire resistance, and R
G
is the
quarter-bridge completion resistance.
Shunt Calibration
The NI 9235/9236 shunt calibration circuitry consists of a
precision resistor and a software-controlled switch, connected
across the internal quarter-bridge completion resistor. Refer to the
software help for information about enabling the shunt calibration
switch for the NI 9235/9236. Each input channel has a unique
shunt calibration resistor that can operate independently, as shown
in Figures 5 and 6.