Resistance pressure sensors with dual resistive elements and four wires are widely used in various pressure monitoring applications. When the pressure increases, the resistance of one resistor increases, and the other resistor decreases. Precise measurements with resistance sensors need to compensate for the loss of line resistance, especially when the line length reaches tens of meters.
The microcontroller or computer can calculate the resistance of the sensor through the differential voltage on the sensor elements RS1 and RS2. Resistors RA, RREF, and RB and the sensor resistance limit the current through RS1 and RS2. The circuit uses AD7705ADC of Analog Devices Company to measure the sensor value. It has three pseudo differential input terminals and provides 16-bit resolution. In this application, AD7705 works in buffer mode, that is, the input bias current is less than 1nA. In the buffer mode, the analog input can withstand large source impedance, but the absolute input voltage value must be 50mV from the ground level, until the drain-drain voltage minus 1.5V. Resistor RB provides an input common mode voltage.
The measurement depends on the value of the reference resistor RREF. To get the highest accuracy, the tolerance of RREF must be 0.01%, and there must be a small temperature coefficient. In order to avoid the self-heating of the sensor, the excitation current should be provided in pulse mode; the software controls the width of the pulse through Q1.
AIN1+, AIN1-calculate RL;
AIN2+, AIN2-calculate RS2+RL;
AIN1-, AIN2- calculate RS1+RS2+RL.
Resistance RS1 and RS2 can be calculated by subtraction. AD7705 has a PGA (Programmable Gain Amplifier) for amplifying small input signals. The device has self-calibration and system calibration options, which eliminates the gain error and offset error in the device or system.
The pressure measurement depends on the resistance ratio and the temperature in the equation P=F (RS1/RS2, T). The parameter T is a compensation coefficient of the temperature dependence of the resistance sensor, T=F (RS1+RS2).