Chopping technique for continuous time sigma-delta ADCs without Q-noise folding
A chopping transconductor includes an transconductor input stage coupled with input signals of the chopping transconductor; a chopping switch coupled with an output of the transconductor input stage, the chopping switch having a switch output; and a cascode transistor, wherein the switch output is coupled to an output of the chopping transconductor through the cascode transistor. The chopping transconductor may be used in an analog-to-digital converter to isolate chopping switches from junctions with quantization noise.
1. A chopping transconductor comprising:
an transconductor input stage coupled with input signals of the chopping transconductor;
a chopping switch coupled with an output of the transconductor input stage, the chopping switch having a switch output;
a first output cascode transistor, wherein the switch output is coupled to a chopping transconductor output through the first output cascode transistor; and
the switch output includes a first switch output and a second switch output and the chopping transconductor output includes a first chopping transconductor output and a second chopping transconductor output.
2. The chopping transconductor of claim 1 , wherein the output of the transconductor input stage includes a first transconductor stage output and a second transconductor stage output.
3. The chopping transconductor of claim 1 , wherein:
the first switch output is coupled through a first cascode transistor to the first chopping transconductor output, and the second switch output is coupled through a second cascode transistor to the second chopping transconductor output.
4. The chopping transconductor of claim 1 , further comprising:
an input chopping switch coupled between the input signals and the transconductor input stage.
5. The chopping transconductor of claim 1 , further comprising:
a parallel chopping switch coupled with the chopping transconductor output.
6. The chopping transconductor of claim 5 , wherein an output of the parallel chopping switch is coupled through a second output cascode transistor to the chopping transconductor output.
7. The chopping transconductor of claim 6 , wherein the output of the parallel chopping switch includes a first switch output and a second switch output and the chopping transconductor output includes a first chopping transconductor output and a second chopping transconductor output.
8. The chopping transconductor of claim 7 , wherein:
the first switch output is coupled through a first cascode transistor to the first chopping transconductor output, and the second switch output is coupled through a second cascode transistor to the second chopping transconductor output.
9. The chopping transconductor of claim 1 , wherein the chopping switch includes a plurality of transistors of a same junction type as the first output cascode transistor.
10. The chopping transconductor of claim 6 , wherein the parallel chopping switch includes a plurality of transistors of a same junction type as the second output cascode transistor.
11. A method of processing a signal in an analog-to-digital converter, comprising:
receiving an input signal,
amplifying the input signal to produce a first output signal,
chopping the first output signal to produce a second output signal;
coupling the second output signal through an output cascode transistor to a summing junction;
coupling a quantization feedback signal to the summing junction; and
the step of chopping is performed using a chopping switch and coupling the second output signal includes:
coupling a first output of the chopping switch to a source of a first cascode transistor;
coupling a drain of the first cascode transistor to the summing junction;
coupling a second output of the chopping switch to a source of a second cascode transistor; and
coupling a drain of the second cascode transistor to the summing junction.
12. The method of claim 11 , wherein receiving the input signal includes chopping the input signal before amplifying the input signal.
13. An analog to digital converter comprising:
a chopping transconductor comprising:
an transconductor input stage coupled with input signals of the chopping transconductor;
a chopping switch coupled with an output of the transconductor input stage, the chopping switch having a switch output; and
a first output cascode transistor, wherein the switch output is coupled to a chopping transconductor output through the first output cascode transistor;
a summing junction coupled with the chopping transconductor output; and
a quanitizer providing a feedback signal coupled with the summing junction.
14. The analog to digital converter of claim 13 , wherein the switch output includes a first switch output and a second switch output and the chopping transconductor output includes a first chopping transconductor output and a second chopping transconductor output.
15. The analog to digital converter of claim 14 , wherein:
the first switch output is coupled through a first cascode transistor to the first chopping transconductor output, and the second switch output is coupled through a second cascode transistor to the second chopping transconductor output.
16. The analog to digital converter of claim 13 , further comprising:
a parallel chopping switch coupled with the chopping transconductor output.
17. The analog to digital converter of claim 16 , wherein an output of the parallel chopping switch is coupled through a second output cascode transistor to the chopping transconductor output.
18. The analog to digital converter of claim 17 , wherein the output of the parallel chopping switch includes a first switch output and a second switch output and the chopping transconductor output includes a first chopping transconductor output and a second chopping transconductor output; wherein the first switch output is coupled through a first cascode transistor to the first chopping transconductor output, and the second switch output is coupled through a second cascode transistor to the second chopping transconductor output.