Measurement of the response of resistive sensors using logic oscillators and integrated frequency measurement

A simple method for the interrogation of resistive sensors is presented together with its application to the measurement of conducting polymer sensors. The noise present in the system was analysed using a precision fixed resistor and a conducting polymer sensor exposed to varying levels of humidified air. The sensitivity of the technique was found to be equivalent to other direct current (dc) measurement techniques. A technique to measure frequency changes is also presented where the main component is a field programmable gate array (FPGA). The noise in this system was compared to that of a commercial counter and found to be only six times greater at +/-0.001% (1 standard deviation). (C) 1999 Elsevier Science B.V. All rights reserved.