Randomly sampling reference ADC for calibration
Analog-to-digital converters (ADCs) can have errors which can affect their performance. To improve the performance, many techniques have been used to compensate or correct for the errors. When the ADCs are being implemented with sub-micron technology, ADCs can be readily and easily equipped with an on-chip microprocessor for performing a variety of digital functions. The on-chip microprocessor and any suitable digital circuitry can implement functions for reducing those errors, enabling certain undesirable artifacts to be reduced, and providing a flexible platform for a highly configurable ADC. The on-chip microprocessor is particularly useful for a randomized time-interleaved ADC. Moreover, a randomly sampling ADC can be added in parallel to a main ADC for calibration purposes. Furthermore, the overall system can include an efficient implementation for correcting errors in an ADC.
1. A randomized interleaved analog-to-digital converter comprising:
two or more analog-to-digital converters to sample, interleaved in time according to a pseudo-randomized sequence, an analog input of the randomized interleaved analog-to-digital converter and generate respective digital outputs;
a digital combiner to combine the respective digital outputs of the two or more analog-to-digital converters to generate a digital output of the randomized interleaved analog-to-digital converter based on the pseudo-randomized sequence;
a reference analog-to-digital converter for randomly sampling the analog input and generating reference digital outputs that are representative of the digital outputs of the two or more analog-to-digital converters based on randomized sampling periods; and
calibration logic for measuring interleaving errors of the two-or analog-to-digital converters based on the respective digital outputs of the two-or more analog-to-digital converters and the reference digital outputs.
2. The randomized interleaved analog-to-digital converter of claim 1 , wherein each time the reference analog-to-digital converter samples the analog input, the reference analog-to-digital converter samples the analog-input along with one of the analog-to-digital converters that is sampling the analog input.
3. The randomized interleaved analog-to-digital converter of claim 1 , wherein the reference analog-to-digital converter has as sampling frequency which is spread out across a range of frequencies.
4. The randomized interleaved analog-to-digital converter of claim 1 , further comprising:
a clock generator for generating a clock signal having a range of clock periods for the reference analog-to-digital converter to randomly sample the analog input.
5. The randomized interleaved analog-to-digital converter of claim 1 , further comprising:
one or more clock divider circuits that output an edge of a clock signal for driving the reference analog-to-digital converter to sample the analog input every X number of clock cycles of an input clock; and
a randomization engine to randomize X used by the one or more clock divider circuits.
6. The randomized interleaved analog-to-digital converter of claim 1 , wherein:
a first resolution of any one of the two or more analog-to-digital converters is higher than a second resolution of the reference analog-to-digital converter.
7. The randomized interleaved analog-to-digital converter of claim 1 , wherein the reference analog-to-digital converter serves as an additional signal path for converting the analog input signal in parallel with the two or more analog-to-digital converters.
8. A method for calibrating a randomized interleaved analog-to-digital converter comprising:
generating, by a clock generator, first clock signals for controlling two or more analog-to-digital converters of the randomized time-interleaved analog-to-digital converter to sample, interleaved in time according to a pseudo-randomized sequence, an analog input of the randomized time-interleaved analog-to-digital converter;
generating, by the clock generator, a second clock signal for triggering a reference analog-to-digital converter to randomly sample the analog input and generate digital outputs at randomized periods, wherein, when the reference analog-to-digital converter samples the analog input, one of the two or more analog-to-digital converters is also sampling the analog input at substantially the same time;
processing respective digital outputs of the two or more analog-to-digital converters and the digital outputs of the reference analog-to-digital converter over a period of time to generate measurements; and
determining interleaving errors associated with the two or more analog-to-digital converters based on the measurements.
9. The method of claim 8 , wherein processing respective digital outputs of the two or more analog-to-digital converters and digital outputs of the reference analog-to-digital converter to generate measurements comprises:
for each one of the two or more analog-to-digital converters, determining a mean of differences between digital outputs of a particular analog-to-digital converter and corresponding digital outputs of the reference analog-to-digital converter generated when the reference analog-to-digital converter is sampling along the particular analog-to-digital converter.
10. The method of claim 9 , wherein determining interleaving errors comprises updating an offset correction term for the particular analog-to-digital converter based on the mean.
11. The method of claim 8 , wherein processing respective digital outputs of the two or more analog-to-digital converters and digital outputs of the reference analog-to-digital converter to generate measurements comprises:
for each one of the two or more analog-to-digital converters, determining a mean of differences between magnitudes of digital outputs of a particular analog-to-digital converter and magnitudes of corresponding digital outputs of the reference analog-to-digital converter generated when the reference analog-to-digital converter is sampling along the particular analog-to-digital converter.
12. The method of claim 11 , wherein determining interleaving errors comprises updating an interleaving gain correction term for the particular analog-to-digital converter based on the mean.
13. The method of claim 8 , wherein processing respective digital outputs of the two or more analog-to-digital converters and digital outputs of the reference analog-to-digital converter over a period of time to generate measurements comprises:
for each one of the two or more analog-to-digital converters, determining a cross correlation of a first signal representing digital outputs of a particular analog-to-digital converter and a second signal representing digital outputs of the reference analog-to-digital converter generated when the reference analog-to-digital converter is sampling along the particular analog-to-digital converter.
14. The method of claim 13 , wherein determining interleaving errors comprises updating an interleaving skew correction term for the particular analog-to-digital converter based on the cross correlation of the first signal and the second signal.
15. The method of claim 13 , wherein the digital outputs of the particular analog-to-digital converter and the digital outputs of the reference analog-to-digital converter are represented by a single bit.
16. A randomized converter system on a single chip for converting an analog input to a digital output, the system comprising:
two or more analog-to-digital converters to sample, interleaved in time according to a pseudo-randomized sequence, the analog input of the randomized converter system and generate respective digital outputs;
a reference analog-to-digital converter for sampling the analog input along with one of the two or more analog-to-digital converters at random and generating reference digital outputs according to randomized sampling periods; and
logic for recording measurements of digital outputs of the two or more analog-to-digital converters and the reference digital outputs of the reference analog-to-digital converter; and
an microprocessor on-chip with the two or more analog-to-digital converters for assisting the randomized converter system based on the measurements.
17. The randomized converter system of claim 16 , further comprising:
a clock generator for generating clock signals to the two or more analog-to-digital converters to sample, interleaved in time according to the pseudo-randomized sequence, and a clock signal to the reference analog-to-digital converter for randomly sampling the analog input.
18. The randomized converter system of claim 17 , wherein the clock generator further generates a spread spectrum clock signal for clocking the on-chip microprocessor, wherein instantaneous period of the spread spectrum clock signal is randomized.
19. The randomized converter system of claim 16 , wherein the on-chip microprocessor executes instructions for computing one or more of the following: interleaving offset correction term, interleaving gain correction term, interleaving skew correction term, frequency response, and linearity of any one of the two or more analog -to-digital converters.
20. The randomized converter system of claim 16 , wherein the reference analog-to-digital converter and the two or more analog-to-digital converters are each sampling the analog input according to randomized periods.