Signal dependent subtractive dithering
A sigma-delta modulator circuit selectively removes a dither signal previously added to an input of a quantizer circuit from the quantizer circuit output when addition of the dither signal causes a digital state change in the quantizer circuit output. Various examples for enabling the selective removal of the dither signal are described. In one embodiment, a second quantizer circuit provides a non-dithered output signal for comparison, by a digital comparator, with the dithered output signal. In another embodiment, a single quantizer circuit provides the dithered and non-dithered output signals in turn, for comparison. A subtraction circuit may remove the dither signal as appropriate. Embodiments enable retention of the improved limit cycle tone reduction achievable via dithering while reducing the need for circuits with increased signal headroom, and associated design complexity and power dissipation.
1. An integrated circuit for sigma-delta modulated analog-to-digital conversion, the integrated circuit comprising:
a dithered sigma-delta modulator circuit, including an analog signal input receiving an analog input signal and a dither signal input receiving a dither signal; and
a digital comparator circuit, coupled to the sigma-delta modulator circuit to receive a quantizer output signal, the digital comparator circuit providing an output signal by selectively removing the dither signal from the quantizer output signal when the dither signal causes a digital state change in the quantizer output signal.
2. The integrated circuit of claim 1 wherein the digital comparator circuit performs the removing using a subtraction circuit.
3. The integrated circuit of claim 1 wherein the digital comparator circuit does not remove the dither signal from the quantizer output signal when the dither signal causes no digital state change in the quantizer output signal.
4. The integrated circuit of claim 1 wherein the sigma-delta modulator circuit further includes:
a feedback subtraction circuit subtracting an analog feedback signal from the analog input signal and providing an adjusted input signal to a coupled loop filter circuit, the loop filter circuit providing a loop filter output signal to a coupled adder circuit, the adder circuit adding the dither signal to the loop filter output signal and providing a dithered loop filter output signal to a quantizer circuit, the quantizer circuit quantizing the dithered loop filter output signal and providing the quantizer output signal; and
a digital-to-analog converter circuit converting the output signal to the analog feedback signal.
5. The integrated circuit of claim 4 wherein the quantizer circuit further separately quantizes the loop filter output signal to provide a non-dithered quantizer output signal, a memory circuit stores at least one of the quantizer output signal and the non-dithered quantizer output signal, and the digital comparator circuit compares the quantizer output signal and the non-dithered quantizer output signal.
6. The integrated circuit of claim 4 further comprising a second quantizer circuit that quantizes the loop filter output signal to provide a non-dithered quantizer output signal for comparison with the quantizer output signal to determine if the dither signal causes a digital state change in the quantizer output signal.
7. The integrated circuit of claim 4 wherein the feedback subtraction circuit selectively adds the dither signal to the adjusted input signal when the dither signal causes the digital state change in the quantizer output signal, and wherein the digital-to-analog converter circuit converts the quantizer output signal to the analog feedback signal.
8. The integrated circuit of claim 7 wherein the feedback subtraction circuit does not add the dither signal to the adjusted input signal when the dither signal causes no digital state change in the quantizer output signal.
9. A method for sigma-delta modulation, the method comprising:
adding a dither signal to an analog input of a quantizer circuit in a sigma-delta modulator circuit; and
selectively removing the dither signal from a quantizer output signal provided by the quantizer circuit, when the dither signal causes a digital state change in the quantizer output signal wherein the removing does not remove the dither signal from the quantizer output signal when the dither signal causes no digital state change in the quantizer output signal.
10. The method of claim 9 wherein the removing further includes subtracting the dither signal from the quantizer output signal, using a subtraction circuit.
11. The method of claim 9 further including:
subtracting an analog feedback signal from an analog input of the sigma-delta modulator circuit and providing an adjusted input signal to a loop filter circuit, using a feedback subtraction circuit;
filtering the adjusted input signal and providing a loop filter output signal to an adder circuit, using the loop filter circuit;
quantizing the dithered loop filter output signal and providing the quantizer output signal to a digital-to-analog converter circuit and a digital comparator circuit, using the quantizer circuit; and
converting the quantizer output signal to the feedback signal and providing the feedback signal to the feedback subtraction circuit, using the digital-to-analog converter circuit.
12. The method of claim 11 wherein:
the adding further includes adding the dither signal to the loop filter output signal and providing the dithered loop filter output signal to the quantizer circuit, using the adder circuit; and
the removing further includes modifying the quantizer output signal, using the digital comparator circuit.
13. The method of claim 11 further including:
separately quantizing the loop filter output signal to provide a non-dithered output signal, using the quantizer circuit;
storing at least one of the quantizer output signal and the non-dithered output signal, using a coupled memory circuit; and
comparing the quantizer output signal and the non-dithered quantizer output signal, using the digital comparator circuit.
14. The method of claim 11 further including quantizing the loop filter output signal to provide a non-dithered quantizer output signal for comparison with the quantizer output signal to determine if the dither signal causes a digital state change in the quantizer output signal, using a second quantizer circuit.
15. The method of claim 11 wherein the adding further includes selectively adding the dither signal to the adjusted input signal when the dither signal causes the digital state change in the quantizer output signal.
16. The method of claim 15 wherein the dither signal is not added to the adjusted input signal when the dither signal causes no digital state change in the quantizer output signal.
17. A system for sigma-delta modulation, the system comprising:
means for adding a dither signal to an analog input of a quantizer circuit in a sigma-delta modulator circuit; and
means for selectively removing the dither signal from a quantizer output signal provided by the quantizer circuit, when the dither signal causes a digital state change in the quantizer output signal wherein the means for selectively removing does not remove the dither signal from the quantizer output signal when the dither signal causes no digital state change in the quantizer output signal.