Image sensors having data converters with low noise comparators
An image sensor may include an array of imaging pixels arranged in rows and columns. Each column of pixels can be coupled to a column analog-to-digital converter (ADC) via a pixel output line. The column ADC can include a first low noise single-ended comparison stage, a second low noise single-ended comparison stage, a latch circuit, and a counter. The first low noise single-ended comparison stage may include one or more current source transistors, a voltage ramp generator, a common source amplifier transistor, one or more autozero components, one or more capacitors such as a noise filtering capacitor, and a voltage clamping circuit. The voltage ramp generator can output an increasing voltage ramp or a decreasing voltage ramp.
1. An image sensor comprising
a pixel output line;
a plurality of pixels coupled to the pixel output line; and
a data converter coupled to the pixel output line and having a first single-ended comparator stage configured to receive a ramp voltage signal and a second single-ended comparator stage configured to receive an output signal from the first single-ended comparator stage, wherein the first single-ended comparator stage comprises a common source amplifier transistor having a gate terminal configured to receive the ramp voltage signal and a first source-drain terminal coupled to the pixel output line.
2. The image sensor of claim 1 , further comprising:
a first current source transistor coupled to the pixel output line; and
a second current source transistor coupled between the first current source transistor and the pixel output line.
3. The image sensor of claim 2 , wherein the first single-ended comparator stage comprises a third current source transistor coupled between the second current source transistor and the pixel output line.
4. The image sensor of claim 3 , wherein the common source amplifier transistor further comprises a second source-drain terminal coupled to a node between the second and third current source transistors.
5. The image sensor of claim 1 , wherein the common source amplifier transistor comprises a p-type transistor.
6. The image sensor of claim 4 , wherein the first single-ended comparator stage further comprises an autozero switch coupled between the gate terminal and the second source-drain terminal of the common source amplifier transistor.
7. The image sensor of claim 6 , wherein the first single-ended comparator stage further comprises a first capacitor having a first terminal coupled to the gate terminal of the common source amplifier transistor and having a second terminal configured to receive the ramp voltage signal.
8. The image sensor of claim 7 , wherein the first single-ended comparator stage further comprises a second capacitor having a first terminal coupled to the gate terminal of the common source amplifier transistor and having a second terminal coupled to a power supply line.
9. The image sensor of claim 4 , wherein the first single-ended comparator stage further comprises a filter capacitor having a first terminal coupled to the first source-drain terminal of the common source amplifier transistor and having a second terminal coupled to the second source-drain terminal of the common source amplifier transistor.
10. The image sensor of claim 6 , wherein the first single-ended comparator stage further comprises a voltage clamping circuit directly coupled to the second source-drain terminal of the common source amplifier transistor.
11. The image sensor of claim 10 , wherein the voltage clamping circuit comprises:
a clamping transistor having a source terminal coupled to the second source-drain terminal of the common source amplifier transistor and a gate terminal coupled to a node between the first and second current source transistors; and
a clamp enable switch coupled between a drain terminal of the clamping transistor and a power supply line.
12. The image sensor of claim 1 , further comprising:
a ramp voltage generator configured to generate the ramp voltage signal, the ramp voltage generator being configured to ramp up the ramp voltage signal to perform a sample-and-hold reset (SHR) conversion and to perform a sample-and-hold signal (SHS) conversion.
13. The image sensor of claim 1 , further comprising:
a ramp voltage generator configured to generate the ramp voltage signal, the ramp voltage generator being configured to ramp down the ramp voltage signal to perform a sample-and-hold reset (SHR) conversion and to perform a sample-and-hold signal (SHS) conversion.
14. The image sensor of claim 1 , wherein the data converter further comprises:
a latch circuit configured to receive a signal from the second single-ended comparator stage; and
a counter configured to receive an enable signal from the latch circuit.
15. An analog-to-digital converter operable with an image sensor pixel, comprising:
a first single-ended comparison stage configured to receive a pixel output signal from the image sensor pixel, to receive a ramp voltage signal from a ramp voltage generator, and to generate a corresponding output signal;
a second single-ended comparison stage configured to receive the output signal from the first single-ended comparison stage and configured to output a first value when the output signal is less than a threshold level and a second value when the output signal is greater than the threshold level;
a latch circuit configured to receive a signal from the second single-ended comparator stage; and
a counter configured to receive an enable signal from the latch circuit.
16. The analog-to-digital converter of claim 15 , further comprising:
a first current source transistor coupled between image sensor pixel and a ground line;
a second current source transistor coupled between the image sensor pixel and the first current source transistor; and
a third current source transistor coupled between the image sensor pixel and the second current source transistor.
17. The analog-to-digital converter of claim 15 , further comprising:
a common source amplifier transistor having a first source-drain terminal coupled to the image sensor pixel and having a second source-drain terminal on which the output signal is generated.
18. The analog-to-digital converter of claim 17 , further comprising:
an autozero switch coupled between the second source-drain terminal and a gate terminal of the common source amplifier transistor;
a first capacitor having a first terminal coupled to the gate terminal of the common source amplifier transistor and having a second terminal configured to receive the ramp voltage signal; and
a second capacitor having a first terminal coupled to the gate terminal of the common source amplifier transistor and having a second terminal coupled to a ground line.
19. The analog-to-digital converter of claim 17 , further comprising:
a filter capacitor coupled across the first and second source-drain terminals of the common source amplifier transistor; and
a voltage clamping circuit directly coupled to the second source-drain terminal of the common source amplifier transistor and configured to selectively pull down the output signal.
20. A method of operating an optical sensor, the method comprising:
with a pixel, outputting a pixel signal;
with a ramp voltage generator, outputting an increasing voltage ramp or a decreasing voltage ramp;
with a first single-ended comparator stage, receiving the pixel signal, receiving the increasing voltage ramp or the decreasing voltage ramp, and generating a first output signal;
with a second single-ended comparator stage, comparing the first output signal to a threshold level and generating a second output signal;
enabling a counter based on the second output signal; and
activating a voltage clamp in the first single-ended comparator stage while the counter is enabled.