National Instruments NI USB-9237 Shunt Calibration

As shown in Figure8, the actual br idge 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 kΩ 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.

lea

bridg

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lead

bridg

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