Bipolar multiple ramp digitisers

An integrating analogue-to-digital converter for producing a digital output signal representative of the magnitude of an analogue input signal, the converter comprising:

• an input terminal for receiving the analogue input signal;
• integrating means comprising a differential amplifier having an inverting input, a non-inverting input and an output, and an integrating capacitance negative-feedback connected between said output and said inverting input;
• first switch means for coupling the input terminal to a predetermined input of the differential amplifier during a first time interval, whereby the capacitance is charged to a voltage whose magnitude and polarity are dependent upon the magnitude and polarity of the analogue input signal;
• polarity testing means arranged to receive a signal representative of the voltage across said capacitance and responsive thereto to produce an output signal having a first level representative of one polarity of the voltage across the capacitance and a second level representative of the other polarity of the voltage across the capacitance;
• a source of at least one reference voltage;
• second switch means responsive to the output signal produced by the polarity testing means at the end of the first time interval to couple the reference voltage source to said predetermined input of the differential amplifier if the analogue input signal and the reference voltage are of different polarity, whereby to discharge the capacitance, and to couple the reference voltage source to the other input of the differential amplifier and temporarily to said predetermined input if the analogue input signal and the reference voltage are of the same polarity, whereby to slew the respective voltages at both inputs and the output of the differential amplifier by an amount equal to the reference voltage, while preserving the voltage across the capacitance substantially unchanged, and to subsequently discharge the capacitance, the discharging of the capacitance taking place, in either case, during a second time interval whose end is indicated by a change in the output signal produced by the polarity testing means from one level to the other;
• a source of clock pulses;
• and
• pulse counter means arranged to count clock pulses during the second time interval, whereby to produce a digital output signal representative of the magnitude of the analogue input signal.