Multi-bit sigma-delta modulator with reduced number of bits in feedback path

Patent metadata

Field	Value
Publication number	US-9007247-B2
Application number	US-201213548069-A
Country	US
Kind code	B2
Filing date	Jul 12, 2012
Priority date	Dec 19, 2007
Publication date	Apr 14, 2015
Grant date	Apr 14, 2015

Abstract

A sigma-delta modulator for an ADC, passes an input signal to a loop filter, then to a multi-bit quantizer of the modulator. An output of the quantizer is passed to a digital filter, and a feedback signal is passed back to the loop filter, the feedback signal having fewer bits than are produced by the multi-bit quantizer. The digital filter has an order greater than one in the passband of the sigma-delta modulator.

First claim

The invention claimed is:

1. A sigma-delta modulator comprising:
a summing stage for generating an error signal as a difference between an input signal and a feedback signal;
a loop filter coupled to an output of the summing stage for filtering the error signal;
a multi-bit quantiser coupled to an output of the loop filter for quantising the filtered error signal; and
a digital filter coupled to an output of the multi-bit quantiser;
a feedback path coupling an output of the digital filter to the summing stage for providing the feedback signal to the summing stage, the feedback signal having fewer bits than a number of bits produced by the multi-bit quantiser;
wherein the difference in gain of the digital filter inside and outside of a passband of the sigma-delta modulator is greater than or equal to 20 log 10((2y−1)/(2z−1)) dB, where y is the number of bits of the feedback signal and z is the number of bits of the multi-bit quantiser.

2. The sigma-delta modulator of claim 1 , the digital filter having a gain not exceeding one outside a passband of the digital filter.

3. The sigma-delta modulator of claim 1 , the digital filter having an order greater than one in the passband of the sigma-delta modulator.

4. The sigma-delta modulator of claim 1 , wherein there is no other feedback path coupling the output of the digital filter to an input of the digital filter.

5. The sigma-delta modulator of claim 1 , wherein the frequency response of the combination of the loop filter and the digital filter has a first order roll-off of gain outside of the passband of the sigma-delta modulator.

6. The sigma-delta modulator of claim 1 , comprising a 1-bit quantiser coupled to the output of the digital filter for generating the feedback signal as a 1-bit feedback signal.

7. The sigma-delta modulator of claim 6 , the 1-bit quantiser being arranged to feed back a most significant bit of an output of the digital filter.

8. The sigma-delta modulator of claim 1 , wherein the feedback path includes a digital-to-analogue converter.

9. The sigma-delta modulator of claim 1 , the loop filter comprising any of a continuous time filter, a switched capacitor filter, or a digital filter.

10. The sigma-delta modulator of claim 1 , the loop filter comprising a filter of order greater than one.

11. The sigma-delta modulator of claim 1 , the loop filter comprising an integrator.

12. The sigma-delta modulator of claim 1 , the loop filter and the digital filter having a bandpass amplitude response.

13. An analogue-to-digital converter having the sigma-delta modulator of claim 1 and a further digital filter coupled to the output of the digital filter.

14. A sigma-delta modulator comprising:
a summing stage for generating an error signal as a difference between an input signal and a feedback signal;
a loop filter coupled to an output of the summing stage for filtering the error signal;
a multi-bit quantiser coupled to an output of the loop filter for quantising the filtered error signal;
a digital filter coupled to an output of the multi-bit quantiser; and
a feedback path coupling an output of the digital filter to the summing stage for providing the feedback signal to the summing stage, the feedback signal having fewer bits than a number of bits produced by the multi-bit quantiser,
wherein the frequency response of the digital filter has a substantially flat gain outside of the passband of the sigma-delta modulator.

15. The sigma-delta modulator of claim 14 , the digital filter having a gain not exceeding one outside a passband of the digital filter.

16. The sigma-delta modulator of claim 14 , the digital filter having an order greater than one in the passband of the sigma-delta modulator.

17. The sigma-delta modulator of claim 14 , wherein there is no other feedback path coupling the output of the digital filter to an input of the digital filter.

18. The sigma-delta modulator of claim 14 , wherein the frequency response of the combination of the loop filter and the digital filter has a first order roll-off of gain outside of the passband of the sigma-delta modulator.

19. The sigma-delta modulator of claim 14 , comprising a 1-bit quantiser coupled to the output of the digital filter for generating the feedback signal as a 1-bit feedback signal.

20. The sigma-delta modulator of claim 19 , the 1-bit quantiser being arranged to feed back a most significant bit of an output of the digital filter.

21. The sigma-delta modulator of claim 14 , wherein the feedback path includes a digital-to-analogue converter.

22. The sigma-delta modulator of claim 14 , the loop filter comprising any of a continuous time filter, a switched capacitor filter, or a digital filter.

23. The sigma-delta modulator of claim 14 , the loop filter comprising a filter of order greater than one.

24. The sigma-delta modulator of claim 14 , the loop filter comprising an integrator.

25. The sigma-delta modulator of claim 14 , the loop filter and the digital filter having a bandpass amplitude response.

26. The sigma-delta modulator of claim 14 , wherein the difference in gain of the digital filter inside and outside of the passband of the sigma-delta modulator is greater than or equal to 20 log 10((2y−1)/(2z−1)) dB, where y is the number of bits of the feedback signal and z is the number of bits of the multi-bit quantiser.

27. An analogue-to-digital converter having the sigma-delta modulator of claim 14 and a further digital filter coupled to the output of the digital filter.

28. A method of converting a signal comprising, in a sigma-delta modulator:
generating an error signal as the difference between an input signal and a feedback signal;
filtering the error signal;
quantising the filtered error signal using a multi-bit quantiser;
filtering the quantised and filtered error signal using a digital filter; and
generating the feedback signal from an output signal of the digital filter,
wherein the feedback signal has fewer bits than a number of bits produced by the multi-bit quantiser; and
wherein the difference in gain of the digital filter inside and outside of a passband of the sigma-delta modulator is greater than or equal to 20 log 10((2y−1)/(2z−1)) dB, where y is the number of bits of the feedback signal and z is the number of bits of the multi-bit quantiser.

29. A method of converting a signal comprising, in a sigma-delta modulator:
generating an error signal as the difference between an input signal and a feedback signal;
filtering the error signal;
quantising the filtered error signal using a multi-bit quantiser;
filtering the quantised and filtered error signal using a digital filter; and
generating the feedback signal from an output signal of the digital filter,
wherein the feedback signal has fewer bits than a number of bits produced by the multi-bit quantiser; and
wherein the frequency response of the digital filter has a substantially flat gain outside of the passband of the sigma-delta modulator.

Assignees

• Van Veldhoven Robert Henrikus Margaretha
• St Ericsson Sa

Inventors

• Van Veldhoven Robert Henrikus Margaretha

Classifications

• H03M3/452

  with weighted feedforward summation, i.e. with feedforward paths from more than one filter stage to the quantiser input · CPC title

• H03M7/302

  characterised by the number of quantisers and their type and resolution · CPC title

• H03M3/428 Primary

  with lower resolution, e.g. single bit, feedback · CPC title