Analog frontend for CCD/CIS sensor
A system for signal processing comprising a cyclic analog to digital converter structure having a first stage and a second stage, wherein the first stage is configured to receive an input signal to perform 1.5 bits/stage ADC and to generate a first stage output signal, and the second stage is configured to receive the first stage output signal and to perform fine offset tuning using a final conversion phase. The second stage further configured to perform 1.5 bits/stage ADC and to generate a second stage output that is fed back to the first stage to iteratively generate a next 1.5 bits, until (N−3) most significant bits of N bits of data are generated. A third stage configured to generate a three least significant bits of the N bits of data using a flash ADC sampling circuit that samples a residue signal at the output of the first stage.
1. A system for signal processing comprising:
a cyclic analog to digital converter (ADC) structure having a first stage and a second stage, wherein the first stage is configured to receive an input signal to perform 1.5 bits/stage ADC and to generate a first stage output signal, and the second stage is configured to receive the first stage output signal and to perform fine offset tuning using a final conversion phase;
the second stage further configured to perform 1.5 bits/stage ADC and to generate a second stage output that is fed back to the first stage to iteratively generate a next 1.5 bits, until (N−3) most significant bits (MSBs) of N bits of data are generated; and
a third stage configured to generate a three least significant bits (LSBs) of the N bits of data using a flash ADC sampling circuit that samples a residue signal at the output of the first stage.
2. The system of claim 1 further comprising a clamp buffer coupled to the input, the clamp buffer configured to limit an amplitude of the input signal to a predetermined maximum amplitude.
3. The system of claim 1 wherein the first stage further comprises a capacitor digital to analog converter (CDAC) configured to receive the input signal and to perform offset correction processing.
4. The system of claim 1 wherein the CDAC comprises:
a first four position switch having a V REFP input, a V REFN input, a V CM input and a V INP input; and
a first capacitor selectively coupled to one of the V REFP input, the V REFN input, the V CM input and the V INP input of the first four position switch and an output of the CDAC.
5. The system of claim 4 wherein the CDAC comprises:
one or more additional four position switches, each having a V REFP input, a V REFN input, a V CM input and a V INP input; and
one or more additional capacitors, each selectively coupled to one of the V REFP input, the V REFN input, the V CM input and the V INP input of one of the additional four position switches and an output of the CDAC.
6. The system of claim 1 wherein the first stage further comprises a programmable gain amplifier (PGA) configured to receive the input and to perform programmable gain amplification processing.
7. The system of claim 3 wherein the first stage further comprises a programmable gain amplifier (PGA) coupled to the CDAC and configured to receive an output of the CDAC and to perform the programmable gain amplification processing on the output of the CDAC.
8. The system of claim 6 wherein the PGA comprises:
a scaling capacitor;
a first capacitor having a first end coupled to the input and a second end coupled to the scaling capacitor; and
a plurality of additional capacitors, each having a first end coupled to a first end of one of a corresponding plurality of switches and a second end coupled to the scaling capacitor, wherein each of the plurality of switches each has a second end coupled to the input and a control terminal configured to receive a control signal and to open or close in response to the control signal.
9. A system for signal processing comprising:
a first phase operated switch coupled to an input;
a second phase operated switch coupled to a reference voltage;
a first capacitor digital to analog converter (CDAC) coupled to the first phase operated switch;
a second CDAC coupled to the second phase operated switch;
an amplifier having a first input coupled to an output of the of the first CDAC and a second input coupled to an output of the second CDAC;
a third phase operated switch coupled between a first output of the amplifier and the first input of the amplifier;
a first programmable gain amplifier (PGA) coupled between the first output of the amplifier and the first input of the amplifier;
a fourth phase operated switch coupled between a second output of the amplifier and the second input of the amplifier;
a second PGA coupled between the second output of the amplifier and the second input of the amplifier; and
a controller configured to operate the first phase operated switch and the second phase operated switch in a first phase, and the third phase operated switch and the fourth phase operated switch in a second phase that is different from the first phase.
10. The system of claim 9 further comprising an analog to digital converter coupled to the first phase operated switch and the second phase operated switch.
11. The system of claim 9 further comprising an analog to digital converter coupled to the first output of the amplifier and the second output of the amplifier.
12. The system of claim 11 further comprising a flash analog to digital converter coupled to the first output of the amplifier and the second output of the amplifier.
13. The system of claim 9 further comprising:
a fifth phase operated switch coupled to the first output of the amplifier and a third CDAC;
a sixth phase operated switch coupled to the second output of the amplifier and a fourth CDAC; and
the controller configured to operate the fifth phase operated switch and the sixth phase operated switch in a third phase that is different from the first phase and the second phase.
14. The system of claim 13 further comprising a second amplifier having a first input coupled to an output of the third CDAC and a second input coupled to an output of the fourth CDAC.
15. The system of claim 14 further comprising a seventh phase operated switch coupled between an output of the second amplifier and the first input of the second amplifier.
16. The system of claim 14 further comprising:
a seventh phase operated switch coupled between a first output of the second amplifier and the first input of the second amplifier; and
an eighth phase operated switch coupled between a second output of the second amplifier and the second input of the second amplifier.
17. The system of claim 14 further comprising:
a seventh phase operated switch coupled between a first output of the second amplifier and the first input of the second amplifier;
an eighth phase operated switch coupled between a second output of the second amplifier and the second input of the second amplifier; and
the controller configured to operate the seventh phase operated switch and the eighth phase operated switch in the third phase.
18. The system of claim 14 further comprising a third PGA coupled between an output of the second amplifier and the first input of the second amplifier.
19. The system of claim 14 further comprising:
a third PGA coupled between a first output of the second amplifier and the first input of the second amplifier; and
a fourth PGA coupled between a second output of the second amplifier and the second input of the second amplifier.
20. The system of claim 13 further comprising:
a seventh phase operated switch coupled between a first output of the second amplifier and the first CDAC; and
an eighth phase operated switch coupled between a second output of the second amplifier and the second CDAC.