A circuit for generating a digital output signal from an analog input signal applied to the circuit, including: first and second capacitors; first means including a switching transistor for charging the first and second capacitors whereby the transistor switches the first capacitor out of the first charging means when a voltage across the first capacitor approaches the voltage of the analog input signal; and a comparison means including a comparator having first and second inputs thereto and an output therefrom. A second means is also included for supplying a first predetermined voltage to the first input which generates a first state at the output of the comparator, the first charging means also being effective to raise the voltage level at the second input after the first capacitor is switched out of the first charging means by charging said second capacitor to cause comparator to switch to a second state. A third means responsive to the second state of the comparator is included for supplying a second predetermined voltage to the first input and also for partially discharging the second capacitor alone and thereafter at least partially discharging the first and second capacitors together so that the output of the comparator toggles or switches back to the first state. The output of the comparator toggles between the first and second states at a frequency which is dependent upon the voltage of the analog input signal.
A method of digitizing an input analog current signal such as a demodulated servo position error signal in a disk file includes charging a capacitor with the input demodulated signal, and comparing the resulting capacitor voltage with a plurality of thresholds to produce a comparator output signal for each threshold exceeded by the capacitor voltage. The comparator output signals are sampled, and each sampling period a current corresponding to the thresholds exceeded is generated. The generated current is used to draw off charge from the capacitor until an equilibrium voltage is reached. By knowing the current generated to remove from the capacitor the voltage placed thereon by the input current signal, then the total charge applied to the capacitor can be determined. Further digital processing of the input signal value may be done without the use of a complicated digital to analog converter.
