Specifications

Section 4. Sensor Support
4-17
measurements are appropriate. Program Code EXAMPLE 4.3-1 shows
CR1
000 code for measuring and processing four wire full bridge circuits.
All bridge measurements have the option (RevEx) to make one set of
measurements with the excitation as programmed and another set of
measurements with the excitation polarity reversed. The offset error in the two
measurements due to thermal EMFs can then be accounted for in the
processing of the measurement instruction. The excitation channel maintains
the excitation voltage or current until the hold for the analog to digital
conversion is completed. When more than one measurement per sensor is
necessary (four wire half bridge, three wire half bridge, six wire full bridge),
excitation is applied separately for each measurement. For example, in the
four-wire half-bridge, when the excitation is reversed, the differential
measurement of the voltage drop across the sensor is made with the excitation
at both polarities and then excitation is again applied and reversed for the
measurement of the voltage drop across the fixed resistor.
Calculating the resistance of a sensor that is one of the legs of a resistive bridge
requires additional processing following the bridge measurement instruction.
FIGURE 4.3-1 lists the schematics of typical bridge configurations and the
calculations
necessary to compute the resistance of any single resistor,
provided the values of the other resistors in the bridge circuit are known.