EFFECTS OF PROCESS VARIATIONS IN A CMOS CIRCUIT FOR TEMPERATURE COMPENSATION OF PIEZORESISTIVE PRESSURE SENSORS

A CMOS circuit has been developed to compensate the sensitivity shift due to temperature changes in silicon pressure sensors. The function of the circuit is to increase the excitation voltage with increasing temperatures. It is of interest to examine the difference between the expected and measured temperature coefficients of the excitation voltage to evaluate the need for individual calibration of circuits and sensor elements. In this paper the effects of process variations are discussed, and it is shown that offset voltages of the operational amplifiers give the most dominant effects. Measurement results from two different types of piezoresistive pressure sensors are given, the SP 80 sensor from SensoNor and the P 10 sensor from Philips. Depending on the sensor type and with a nominal value of one adjustable parameter in the CMOS circuit, the sensitivity shifts of the sensor elements are reduced by a factor of two to six compared to the shifts observed with a constant-voltage excitation. However, with an individual calibration of each circuit/sensor element, process variations from unit to unit can be compensated. In this case, depending on the sensor type, the sensitivity shift can be reduced by a factor of 35-50 over the temperature range -30 to +100 degrees C.