Measurement Computing 6224 - Manual (Page 17)

6224 User’s Manual Pinouts and Connectors 3-3
In the preceding figure, 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 as follows:
Error in Full-bridge sensors = 2R
Lead
/R
Bridge
Error in Half-bridge sensors = R
Lead
/R
Bridge
If the remote sense (RS) signals are connected directly to the bridge resistors, then the 6224 senses
the actual bridge voltage sense and eliminates the gain errors caused by the resistance of the EX+ and EX– leads.
Shunt Calibration
The second mechanism [aside from remote sensing] used to correct errors from wire resistance is Shunt
Calibration. This involves simulating the input of strain by changing the resistance of an arm in the bridge.
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.
Shunt calibration can be used to correct for errors from the resistance of both the excitation wiring and
wiring in the individual resistors of the bridge. Shunt calibration is most critical when measurements are
made on Quarter-Bridge sensors because there is no means to remotely sense around any IR drops in the
connection wiring.
The 6224 includes a precision 100kΩ resistor and a software-controlled switch for each channel. 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 6224 to the sensor, shunt
calibration corrects for these errors and for errors caused by wire resistance within an arm of the bridge.
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 and corrections for offset errors.
Internal Excitation Voltage Sources
The 6224 houses three internal voltage sources; each serves 4 channels. The first source applies excitation voltage to
CH1 through CH4, the second to CH5 through CH8, and the third to CH9 through CH12.
The sensor industry does not recognize a single standard excitation voltage level. However, excitation voltage levels
residing in the range of 2.5V to 10 V are common. Encore’s selections for internal excitation are 2.5V, 3.3V, 5V,
and 10V, and each of the three internal voltage sources can provide up to 150 mW of excitation power. The 6224
automatically reduces internal excitation voltages, as needed, to stay below 150mW.
Since channels are associated by groups of four, the excitation setting applied to one channel in a group will also be
applied to the other channels in that group. As an example: if channel 1 is set to have and internal excitation of 3.3V,
then channels, 2, 3, and 4 will also have 3.3V excitation.
The 6224 measures the ratio of bridge output to bridge excitation, and as such does not require accurate excitation
voltage. For this reason the excitation voltage is not precisely regulated and may vary as much as
10% from the requested voltage, while still achieving accurate measurements.