Voltage controlled oscillator having ratiometric and temperature

Abstract

A voltage controlled oscillator for the conversion of a ratiometric analog signal into a digital signal is disclosed. The oscillator generates a square wave whose frequency is a function of the magnitude of the analog signal input. The variable frequency wave is generated by a comparator receiving high and low threshold levels from a network for comparison with a ramping voltage developed by a timing element. An increasing ramp is provided by a current source whose output is proportional to the analog signal and a decreasing ramp rate is provided by a current sink whose output is similarly proportional to the analog input signal. Because the analog input signal is ratiometric with variations in the power supply voltage, the ramp rates change with these variations, however the high and low threshold levels also change with variations in the supply voltage thereby the comparator generates a square wave output at a frequency independent of these variations. Temperature compensation is provided for the voltage controlled oscillator by matching the temperature coefficient of one of the gain elements in the current source to the temperature coefficient of the timing element. Further temperature compensation is implemented by equalizing the temperature coefficients of the four resistances forming the high and low threshold level network.