Method and apparatus for operating a pipelined ADC circuit
An analog-to-digital converter (ADC) circuit that converts an analog input signal into a digital output circuit includes a noise shaping first stage cascaded with a pipelined second stage. The first stage includes a sample-and-hold circuit and a first order modulator, where the first order modulator includes a noise shaping filter, a FLASH ADC and a feedback DAC. A digital dither generator is used to provide a dither signal to the ADC circuit. The second stage includes a switching circuit and an ADC. A calibration filter connected to the second stage calibrates the ADC circuit. A first reconstruction filter and a second reconstruction filter are used to recombine outputs of the first stage and the second stage of the ADC circuit. The ADC circuit allows high resolution analog-to-digital conversion at a low over-sampling rate and low power dissipation levels.
1. An analog-to-digital converter system for converting an analog input signal to a digital output signal, the system comprising:
a first stage analog-to-digital converter circuit adapted to convert the analog input signal into a first stage digital output;
a digital signal generator adapted to generate a digital test signal, wherein the digital signal generator is further adapted to change an amplitude of the digital test signal based on at least one of a polarity and an amplitude of the analog input signal;
a combiner coupled to the first stage analog-to-digital converter circuit and adapted to combine the digital test signal with the digital output of the first stage analog-to-digital converter circuit;
a residue signal generator adapted to receive the output of the combiner and to generate a residue signal using the output of the combiner and the analog input signal;
a noise shaping circuit adapted to noise shape the residue signal;
a sampling circuit coupled to the output of the noise shaping circuit and adapted to sample the noise shaped residue signal;
a second stage analog-to-digital converter circuit coupled to the output of the sampling circuit and adapted to generate a second stage digital output based on the sampled residue signal; and
a digital correction circuit coupled to the output of the first stage analog-to-digital converter circuit and to the output of the second stage analog-to-digital converter circuit and adapted to combine the first stage digital output and the second stage digital output using a reconstruction filter.
2. An analog-to-digital converter system of claim 1 , wherein the first stage analog-to-digital converter circuit operates at a first operating frequency and the second stage analog-to-digital converter circuit operates at a second operating frequency, wherein the second operating frequency is a fraction of the first operating frequency.
3. An analog-to-digital converter system of claim 2 , wherein the second operating frequency is equal to a Nyquist frequency of the sampling circuit and the first operating frequency is at least twice the Nyquist frequency of the sampling circuit.
4. An analog-to-digital converter system of claim 1 , wherein the digital signal generator is further adapted to change an amplitude of the digital test signal in a manner so as to prevent the second stage analog-to-digital converter circuit from saturating.
5. An analog-to-digital converter system of claim 1 , wherein the digital correction circuit further includes a decimation filter adapted to cancel the effect of the digital test signal from the combination of the first stage digital output and the second stage digital output.
6. An analog-to-digital converter system of claim 1 , wherein the second stage analog-to-digital converter circuit is one of a pipelined analog-to-digital converter circuit or a cyclic analog-to-digital converter circuit and wherein the first stage analog-to-digital converter circuit is a delta sigma analog-to-digital converter circuit.
7. An analog-to-digital converter system of claim 1 , wherein the first stage analog-to-digital converter circuit further includes:
a multiplier coupled to the output of the noise shaping circuit and adapted to multiply the output of the noise shaping circuit;
a feed-forward path adapted to feed-forward the analog input signal to a summer circuit;
the summer circuit adapted to combine the analog input signal and the output of the multiplier; and
a quantizer adapted to generate a quantized output signal based on the combined output of the summer circuit.
8. An analog-to-digital converter system of claim 7 , wherein the quantizer is further adapted to introduce an offset into the quantized output signal.
9. An analog-to-digital converter system of claim 8 , further comprising an offset correction circuit adapted to correct the offset introduced by the quantizer.
10. An analog-to-digital converter system of claim 1 , wherein the second stage analog-to-digital converter circuit includes a calibration filter adapted to correct gain errors and pole errors in the first stage digital output.
11. A method of converting an analog input signal to a digital output signal, the method comprising:
generating a first stage digital output based on the analog input signal;
generating a digital test signal, wherein the amplitude of the digital test signal changes based on at least one of a polarity and an amplitude of the analog input signal;
combining the first stage digital output with the digital test signal;
generating a residue signal from the combination of the first stage digital output and the digital test signal;
noise-shaping the residue signal;
sampling the noise shaped residue signal at a second operating frequency;
generating a second stage digital output signal based on the sampled residue; and
combining the first stage digital output and the second stage digital output in a manner so as to cancel the effect of the digital test signal.
12. A method of claim 11 , wherein generating the first stage digital output includes generating the first stage digital output using a first stage analog-to-digital converter circuit operating at a first frequency, and wherein the second operating frequency is a fraction of the first operating frequency.
13. A method of claim 11 , wherein the second operating frequency is equal to a Nyquist frequency of a sampling circuit used for sampling the noise shaped residue signal and the first operating frequency is at least twice the Nyquist frequency of a sampling circuit.
14. A method of claim 11 , wherein noise-shaping the residue of the first stage analog-to-digital converter circuit further includes providing a gain to the residue of the first stage analog-to-digital converter circuit.
15. A method of claim 11 , further comprising changing the amplitude of the digital test signal in a manner so as to prevent the second stage analog-to-digital converter circuit from saturating.
16. A method of claim 11 , further comprising:
multiplying the noise shaped residue signal;
combining the analog input signal to the multiplied noise shaped residue signal; and
generating a quantized output signal based on the combination of the analog input signal and the multiplied noise shaped residue signal.
17. A method of claim 16 , further comprising:
introducing an offset into the quantized output signal; and
correcting the offset introduced by the quantizer from the combination of the first stage digital output and the second stage digital output.
18. A method of claim 11 , wherein combining the first stage digital output and the second stage digital output includes combining the first stage digital output and the second stage digital output using one of(1) software; (2) hardware or (3) firmware.