Comparator circuits with constant input capacitance for a column-parallel single-slope adc

Abstract

Comparator circuits suitable for use in a column-parallel single-slope ADC comprise a comparator, an input voltage sampling switch connected between an input voltage Vand a first node, and a sampling capacitor connected between the first and second nodes and which stores a voltage which varies with Vwhen the sampling switch is closed. A first reset switch is connected between the second node and a reset voltage, an isolation buffer is coupled between the second node and a comparator input, and a voltage ramp switch applies a voltage ramp Vto the first node when closed. The comparator output toggles when Vexceeds V, with the isolation buffer maintaining a nearly constant capacitive load on V. A 'ramp disconnect' feature can be used to increase the circuit's input range, and a dummy capacitor can be employed to maintain a constant capacitance on V.