The difference between the output at full-scale pressure and the output at zero pressure is defined as the span of the pressure sensor. A typical value for the span is 100 mV. A typical value for the resistors in the bridge is 5 kΩ. Loading of the resistor bridge could result in incorrect output voltages of the sensor. Therefore the selection of the circuit configuration, which connects to the sensor, should take into account a minimum loading of the sensor.
The configuration shown in Figure 49 is simple, and is very useful for the read out of pressure sensors. With two operational amplifiers in this application, the dual LMV832 fits very well. The operational amplifier configured as a buffer and connected at the negative output of the pressure sensor prevents the loading of the bridge by resistor R2. The buffer also prevents the resistors of the sensor from affecting the gain of the following gain stage. Given the differential output voltage VS of the pressure sensor, the output signal of this operational amplifier configuration, VOUT, equals Equation 2:
To align the pressure range with the full range of an ADC, the power supply voltage and the span of the pressure sensor are needed. For this example a power supply of 5 V is used and the span of the sensor is 100 mV. When a 100-Ω resistor is used for R2, and a 2.4-kΩ resistor is used for R1, the maximum voltage at the output is 4.95 V and the minimum voltage is 0.05 V. This signal is covering almost the full input range of the ADC. Further processing can take place in the microprocessor following the ADC.