Dual slope analog to digital converter with out-of-range reset

DETAILED DESCRIPTION The invention is a method and apparatus for controlling the operation of a dual slope analog to digital converter such that the state of the converter is reinitialized immediately upon detection of an out-of-range input signal condition.
The method basically comprises sensing the time involved in the signal integration process in the converter, determining a time limit corresponding to the limiting value of the expected range of input signals, and returning the output signal of the integrator to its initial value after the integration process has continued for the time interval corresponding to the limiting value of the expected input signal range.
Apparatus according to the invention basically comprises an integrator having an input terminal and an output terminal, the input terminal being alternately supplied with a signal of unknown value and a signal of known value, the signal of unknown value being supplied for a predetermined time interval whereby the output signal of integrator commences at a first predetermined value and changes at a rate depending on the difference between the unknown input signal value and a reference signal value to a second undetermined value.
Thereafter, the known signal is integrated for an unknown time interval such that the output signal of the integrator has as its starting value the second undetermined value of the output signal and changes at a fixed rate until the predetermined value of the output signal is reached.
A switch is connected across the integrator, the switch being controlled by time interval detection means which senses the time being spent in the integration process and closes the switch after a time interval corresponding to the limiting value of the expected range of input signals.
The time interval detection circuit may comprise a source of clock pulses and a counter which counts the clock pulses occurring during the integration process and produces a signal to close the switch when the count reaches the number corresponding to the maximum value of the expected range of input signals