THE VOLTAGE CONTROLLED DUTY-CYCLE OSCILLATOR: A STABLE MODULATOR FOR INTEGRATED SENSORS (MULTIVIBRATOR)

The Voltage Controlled Duty-Cycle Oscillator (VCDCO) is a modulator which has a squarewave output with a duty-cycle that depends only on the input voltage and temperature. The transfer function of the VCDCO is, to first-order, independent of all component values and matching. This independence is the result of using a ratiometric modulation scheme which exploits the inherent symmetry in the operation of an emitter-coupled multivibrator. Due to the insensitivity to component values, the VCDCO should provide better long term stability and wafer-to-wafer reproducibility than other modulators.;An analysis of the effect of sensing-element noise on the minimum detectable signal of piezoresistive and capacitive integrated pressure sensors is presented. The results of the analysis indicate that the minimum detectable pressure change for piezoresistive integrated pressure sensors is smaller than for comparable capacitive sensors.;A low-power breadboard version of the VCDCO has demonstrated a maximum deviation from linearity of .015% of full scale. When used with a demodulator the overall system had a minimum detectable signal of 6 (mu)V and a maximum error of less than .05% of full scale for an input voltage range of (+OR-)20 mV. A low-power monolithic version of the VCDCO is also described. This implementation can operate from supply voltages as low as 2.5 V, dissipates less than 25 (mu)W with a current output, and uses only nine transistors, seven resistors, and one capacitor. It is used along with a bandgap voltage reference to produce a two-terminal temperature sensor with a duty-cycle modulated output.