Combined programmable gain amplifier and comparator for low power and low area readout in image sensor

1. A switchable amplifier and comparator circuit comprising:

an operational amplifier having an inverting input, a non-inverting input, a first differential output and a second differential output, the first differential output switchably coupled to the inverting input and the second differential output switchably coupled to the non-inverting input;

a first feedback capacitor having a first plate coupled to the inverting input and a second plate switchably coupled to the first differential output;

a second feedback capacitor having a first plate coupled to the non-inverting input and a second plate switchably coupled to the second differential output;

a capacitive load switchably coupled between the first differential output and the second differential output;

a diode clamp circuit switchably coupled between the first differential output and the second differential output; and

a resistive load switchably coupled between the first differential output and the second differential output.

2. The switchable amplifier and comparator circuit of claim 1 further comprising:

a first sampling capacitor coupled between the inverting input and a first switched input node, the first switched input node switchably coupled to a signal input node; and

a second sampling capacitor coupled between the non-inverting input and a second switched input node, the second switched input node switchably coupled to the signal input node and switchably coupled to the first switched input node.

3. The switchable amplifier and comparator circuit of claim 2 wherein:

the second plate of the first feedback capacitor is switchably coupled to a positive ramp signal; and

the second plate of the second feedback capacitor is switchably coupled to a negative ramp signal.

4. The switchable amplifier and comparator circuit of claim 3 wherein:

the first feedback capacitor includes a plurality of individual first feedback capacitors that are programmably connected in parallel with one another; and

the second feedback capacitor includes a plurality of individual second feedback capacitors that are programmably connected in parallel with one another.

5. The switchable amplifier and comparator circuit of claim 4 wherein:

the plurality of individual first feedback capacitors that are programmably connected in parallel with one another have equal capacitance; and

the plurality of individual second feedback capacitors that are programmably connected in parallel with one another have capacitance values that are weighted with respect to one another.

6. The switchable amplifier and comparator circuit of claim 4 wherein the plurality of individual first feedback capacitors are programmably connected in parallel with one another through passgates and the plurality of individual second feedback capacitors are programmably connected in parallel with one another through passgates.

7. A method for downloading data from an imaging array into a single output circuit comprising a differential amplifier switchably configurable to be one of a correlated double sampling programmable gain amplifier and a comparator, the method comprising:

switchably configuring a capacitive load, a diode clamp, and a resistive load coupled between first and second outputs of the differential amplifier to be a correlated double sampling programmable gain amplifier and a comparator;

sampling a pixel sensor output from a column of the array;

sampling a reset state output from the column of the array;

generating a programmable gain correlated double signal sampled signal from the sampled pixel sensor output and the sampled reset state output; and

switchably configuring the capacitive load, the diode clamp, and the resistive load coupled between the first and second outputs of the differential amplifier as a comparator and comparing the programmable gain correlated double signal sampled signal with a ramp signal.