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National Instruments Corporation
13
NI USB-9237 User Guide and Specifications
As shown in Figure 8, the actual bridge excitation voltage is smaller than
the voltage at the EX+ and EX– leads. If remote sensing of the actual bridge
voltage is not used, the resulting gain error is
for full-bridge sensors, and
for half-bridge sensors.
If the remote sense (RS) signals are connected directly to the bridge resistors,
the NI USB-9237 senses the actual bridge voltage sense and eliminates the
gain errors caused by the resistance of the EX+ and EX– leads.
Shunt Calibration
Shunt calibration can correct for errors from the resistance of both the
excitation wiring and wiring in the individual resistors of the bridge. Shunt
calibration is most useful with quarter-bridge sensors because there may be
significant resistance in the wiring to the active resistor in the bridge.
The NI USB-9237 shunt calibration circuitry consists of a precision
100 k
Ω
resistor and a software-controlled switch. You can leave the shunt
calibration terminals connected to the sensor, and then apply or remove the
shunt calibration resistance in software.
While remote sensing corrects for resistances from the EX terminals on the
NI USB-9237 to the sensor, shunt calibration corrects for these errors and
for errors caused by wire resistance within an arm of the bridge.
Shunt calibration involves simulating the input of strain by changing the
resistance of an arm in the bridge by some known amount. This is
accomplished by shunting, or connecting, a large resistor of known value
across one arm of the bridge, creating a known strain-induced change in
resistance. The output of the bridge can then be measured and compared to
the expected voltage value. The results are used to correct gain errors in the
entire measurement path, or to simply verify general operation to gain
confidence in the setup.
A stable signal, which is typically the unloaded state of the sensor, is used
first with the shunt calibration switch off and then again with the switch on.
The difference in these two measurements provides an indication of the
gain errors from wiring resistances. You can design the software
application to correct subsequent readings for this gain error.
2
R
lead
R
bridge
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R
lead
R
bridge
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