A/D Converter using Voltage to Time Conversion

An analog signal can be converted into a digital signal by counting the pulses from a variable frequency source whose frequency is dependent on the analog input signal value. The counting is done for a fixed period of time. Alternatively, the pulses from a fixed frequency source can be countered for a variable period of time, and the time period is then dependent on the analog signal under conversion.

The above figure shows that such an A/D converter. It employs an integrator, a Sample-and-Hold (S/H) circuit, a voltage comparator, and a high-speed counter. A negative reference voltage VR is applied to the integrator which integrates the voltage VR and provides a positive polarity output. The analog signal input under conversion Vi(t) is sampled at a rate fixed by the control voltage Vc and the sampled signal at any instant Vi is applied as input to the non-inverting terminal of the comparator. The integrator output Vs is connected to the inverting input of the comparator.