Analog to digital converter calibration via synchronous demodulation
A technique for dynamically calibrating a successive approximation charge to digital converter by toggling at least some portion of the converter between two predetermined states, with the design goal of balancing the voltage and/or charge that is output in the two states. The two states are chosen such that they are expected to generate the same output voltage when the converter is in “normal” operation mode, e.g., within a fraction of the Least Significant Bit (LSB) resolution of the converter. If there is an imbalance, switching between the two calibration states invariably generates a square wave signal that toggles between two distinct values. A synchronous demodulator having a bandwidth centered at the toggle frequency can then be used to accurately detect an amount of error, which is then feedback to generate correction signals. If there are undesirable static offsets introduced by the synchronous demodulator or by the signal and/or charge levels output by the two differential halves of the converter, a properly timed latch can be used to further stabilize the error signal.
1. A digital converter apparatus comprising:
a converter core having at least two predetermined calibration states in a calibration mode, each of the predetermined calibration states providing a respective predetermined output signal;
a control circuit for switching the converter core between at least two of the predetermined calibration states;
a sampler for providing a converter output signal over a sequence of multiple predetermined calibration states;
a synchronous demodulator for demodulating the converter output signal
wherein the control circuit further switches between the calibration states at a state toggle frequency, and a bandwidth of the synchronous demodulator is centered at the state toggle frequency.
2. The apparatus of claim 1 additionally comprising:
a correction circuit for using the demodulated converter output signal to provide a correction signal to the converter core.
3. The apparatus claim 1 wherein the digital converter is a successive approximation converter.
4. The apparatus of claim 1 wherein the converter core is a complimentary converter having two conversion signal paths therein, with a first conversion signal
path operating as a plus signal path and a second path operation as a minus signal path.
5. The apparatus of claim 4 wherein
a first one of the predetermined calibration states is provided by applying a first set of calibration inputs to the plus signal path and a second set of calibration inputs to the minus signal path; and
a second one of the predetermined calibration states is provided by applying the same second set of calibration inputs to the plus signal path and the same first set of calibration inputs to the minus signal path.
6. The apparatus of claim 1 wherein the synchronous demodulator further comprises:
an integrator for integrating the converter output signal to provide an error signal.
7. The apparatus of claim 1 wherein each of predetermined calibration states provide nominally the same converter output signal.
8. The apparatus of claim 1 additionally comprising:
a chopper stabilizer connected to stabilize at least one component of the digital converter during an operating mode.
9. The apparatus of claim 8 wherein the chopper stabilizer is additionally coupled to the synchronous demodulator.
10. An apparatus for calibrating a digital converter comprising:
a controller for toggling at least some portion of the converter between at least two predetermined calibration states in a calibration mode, with each of the two predetermined calibration states providing a predetermined output signal;
an output circuit for providing a converter output signal over a sequence of multiple predetermined calibration states;
a demodulator for synchronously demodulating the converter output signal; and
an integrator for integrating the converter output signal to provide an error signal, wherein the error signal is generally a ramp type waveform in the absence of noise.
11. The apparatus of claim 10 wherein offset voltages introduced by the converter superimpose a sawtooth type waveform on the ramp waveform.
12. The apparatus of claim 11 additionally comprising:
a latch for producing the error signal in synchronism with the change of calibration states of the converter.
13. The apparatus of claim 10 wherein the converter is a complimentary converter having two conversion signal paths therein, with a first conversion signal path operating as a plus signal path and a second path operation as a minus signal path.
14. The apparatus of claim 13 wherein a common reference input, Vcm, is fed to the plus signal path and the minus signal path.
15. The apparatus of claim 10 wherein a number of stages used by the digital converter in the calibration mode is at least one stage greater than the number of stages of the converter used in an operation mode.
16. An apparatus for calibrating a digital converter comprising:
a controller for toggling at least some portion of the converter between at least two predetermined calibration states in a calibration mode, with each of the two predetermined calibration states providing a predetermined output signal;
an output circuit for providing a converter output signal over a sequence of multiple predetermined calibration states;
a demodulator for synchronously demodulating the converter output signal;
wherein the converter is a complimentary type converter having a first conversion signal path operating as a plus signal path, and a second conversion signal path operating as a minus signal path; and
wherein the two conversion signal paths are Charge Coupled Device (CCD) pipeline stages.
17. The apparatus of claim 16 wherein calibration inputs are fed to a respective set of two or more adjustable charge splitters, with a charge splitter associated with a corresponding one of the CCD pipeline stages.
18. The apparatus of claim 17 additionally comprising:
an error circuit for generating an error signal from the synchronously demodulated converter output signal; and
a splitter controller for providing adjustment signals to the adjustable splitters from the error signal.