Excess loop delay compensation for a delta-sigma modulator

A circuit comprising:

• a delta-sigma modulator comprising:
• an analog loop filter of order n comprising:
• a first analog loop filter portion having an order n1 of between 1 and p, wherein p is an integer of at least one and of at most n, and
• a second analog loop filter portion having an input coupled to an output of the first analog loop filter portion, the second analog loop filter portion having an order of p-n,
• a quantizer coupled to an output of the second analog loop filter portion, the quantizer having a first excess loop delay (ELD) and configured to provide a quantized sample every sampling period,
• an inner loop portion coupled between an output of the quantizer and an input of the second analog loop filter portion, the inner loop portion having a second ELD, the inner loop portion comprising a finite impulse response (FIR) filter and a first digital-to-analog converter (DAC) coupled between an output of the FIR filter and the input of the second analog loop filter portion,
• a functional circuit coupled between the output of the quantizer and an input of the first analog loop filter portion, and
• a second DAC having an input coupled to the output of the functional circuit and an output coupled to an input of the first analog loop filter portion, wherein the second DAC and the functional circuit have a third ELD, a sum of the first ELD and the second ELD is not larger than one clock cycle of the delta-sigma modulator, and the third ELD is larger than one clock cycle of the delta-sigma modulator, and wherein the FIR filter is configured to correct for an effect of the first ELD, the second ELD, and the third ELD on a noise transfer function of the delta-sigma modulator.

A method of operating a delta-sigma modulator, the method comprising:

• filtering an analog input signal using an analog loop filter, wherein the analog loop filter comprises an outer portion, and an inner portion coupled to an output of the inner portion;
• quantizing an output of the inner portion of the analog loop filter to form a quantized signal using a quantizer;
• feeding back the quantized signal to an input of the outer portion of the analog loop filter via an outer feedback path;
• filtering the quantized signal to form a filtered signal using a compensation filter;
• and
• feeding back the filtered signal to an input of the inner portion of the analog loop filter, wherein:
• the compensation filter has a transfer function configured to correct for an effect of an outer excess loop delay (ELD) and an inner ELD on the delta-sigma modulator,
• the outer ELD comprises a delay from the input of the quantizer to the input of the outer portion of the analog loop filter including the outer feedback path, and the inner ELD comprises a delay from the input to the quantizer to the input of the inner portion of the analog loop filter including at least a portion of the outer feedback path,
• the inner ELD is less than one clock cycle of the delta-sigma modulator, and
• the outer ELD is at least two clock cycles of the delta-sigma modulator.