A Novel, Highly Linear, Voltage and Temperature Independent Sensor Interface using Pulse Width Modulation
A sensor interface based on a ring oscillator is presented. By using the output of a differential sensor to simultaneously increase and decrease the delay of 4 of the 8 stages, a Pulse Width Modulated (PWM) signal can be obtained at the output. The duty cycle of the digital PWM signal is a measure for the sensor value. Since the duty cycle only relies on the ratio of the stage delays and not on the frequency, it does not suffer from temperature or supply voltage changes. A 130 nm CMOS oscillator and output stage was designed to prove the principle. Simulations show a maximum nonlinearity of only 0.07% over a voltage span of 0.9-1.6 V for a +/-25% sensor output. The maximum nonlinearity over a -40 to 120 °C temperature span is 0.19%. The maximum error over the supply voltage and temperature span is below 1.2%.