Successive approximation register (SAR) analog-to-digital converter (ADC), radar unit and method for improving harmonic distortion performance
A successive approximation register, SAR, analog-to-digital converter, ADC, ( 400 ) is described. The SAR ADC ( 400 ) includes: a track and hold circuit ( 414 ) configured to sample an analog input signal ( 410 ); a comparator ( 416 ) coupled to the track and hold circuit and configured to compare the sampled analog input signal ( 410 ) with a DAC ( 444 ) output voltage; and a feedback path ( 422 ) that comprises a digital-to-analog converter, DAC, ( 444 ) configured to generate the reference voltage that approximates the input analog signal ( 410 ). The SAR ADC ( 400 ) further includes a dither circuit ( 468 ) coupled to or located in the feedback path ( 422 ) and arranged to add a dither signal at an input of the DAC ( 444 ) in a first time period and subtract the dither signal from the output digital signal routed via the feedback path ( 422 ) and input of the DAC ( 444 ) in a second time period during a conversion phase of the SAR ADC ( 400 ).
1. A successive approximation register, SAR, analog-to-digital converter, ADC, comprises:
a track and hold circuit configured to sample an analog input signal;
a comparator coupled to the track and hold circuit and configured to compare the sampled analog input signal with a DAC output voltage; and
a feedback path that comprises a digital-to-analog converter, DAC, configured to generate the reference voltage that approximates the input analog signal;
a dither circuit coupled to the feedback path and arranged to add a dither signal at the input of the DAC in a first time period and subtract the dither signal from the output digital signal routed via the feedback path and an input of the DAC in a second time period during a conversion phase of the SAR ADC.
2. The SAR ADC of claim 1 wherein the addition of the dither signal occurs during a tracking phase of the SAR ADC.
3. The SAR ADC of claim 1 wherein the dither circuit is located in the feedback path.
4. The SAR ADC of claim 1 wherein the DAC comprises a number of DAC cells and wherein a SAR controller coupled to an output of the comparator configures the dither circuit to subtract the dither signal from the output digital signal in the second time period after a number (N) of most significant ADC bits have been resolved.
5. The SAR ADC of claim 4 wherein the number (N) of most significant DAC cells is less than a total number of DAC cells to be resolved.
6. The SAR ADC of claim 5 wherein the dither circuit is arranged to successively resolve a preset or programmable number of most significant bit, MSB, values of the sampled analog input signal ( 414 ).
7. The SAR ADC of claim 1 , wherein the subtraction of the dither occurs in an analog domain at an output of the DAC before a final ADC output code is available.
8. The SAR ADC of claim 1 , wherein the dither circuit applies the dither signal with opposite polarity to an input of the DAC input, thereby subtracting a previously added dither value, D, from a present DAC output.
9. The SAR ADC of claim 1 , wherein the dither circuit comprises a random number generator coupled to a summing junction located in the feedback path and configured to add and subsequently subtract a random number value generated as the dither signal to the input of the DAC.
10. The SAR ADC of claim 9 wherein the random number generator generates a positive representation of the dither signal when adding the dither signal to the output digital signal in the summing junction and a negative representation of the dither signal when subtracting the dither signal from the output digital signal in the summing junction.
11. The SAR ADC of claim 1 , wherein the DAC is one from a group of: a current steering, charge redistribution or resistive DAC.
12. A radar unit comprising at least one antenna coupled to at least one radio frequency circuit configured to receive and down-convert received radar signals, wherein the radar unit comprises a baseband circuit having a successive approximation register, SAR, analog-to-digital converter, ADC, arranged to receive a down-converted baseband received radar signal and convert the down-converted baseband received radar signal to a digital form, wherein the SAR ADC comprises:
a track and hold circuit configured to sample an analog input signal;
a comparator coupled to the track and hold circuit and configured to compare the sampled analog input signal with a reference voltage; and
a feedback path that comprises a digital to analog converter, DAC, configured to generate the reference voltage that approximates the input analog signal, such that the comparator determines a sign of a subtraction operation between the sampled input and DAC output;
a dither circuit coupled to the feedback path and arranged to add a dither signal at the input of the DAC in a first time period and subtract the dither signal from the output digital signal routed via the feedback path and input of the DAC in a second time period during a conversion phase of the SAR ADC.
13. A method for digitizing a received analog input signal in a successive approximation register, SAR, analog-to-digital converter, ADC, wherein the method comprises:
sampling an analog input signal by a track and hold circuit;
comparing the sampled analog input signal ( 414 ) with a DAC output voltage by a comparator; and
generating, by a digital to analog converter, DAC, the voltage that approximates the input analog signal;
adding a dither signal at an input of the DAC in a first time period and
subtracting the dither signal from the output digital signal routed via a feedback path and input of the DAC in a second time period during a conversion phase of the SAR ADC.
14. The method of claim 13 wherein adding the dither signal at an input of the DAC in a first time period comprises adding during a tracking phase of the SAR ADC.
15. The method of claim 13 wherein the DAC comprises a number of DAC cells and subtracting the dither signal comprises subtracting the dither signal from the output digital signal in the second time period after a number (N) of most significant DAC cells have been resolved.
16. The method of claim 15 , wherein the number (N) of most significant DAC cells is less than a total number of DAC cells to be resolved.
17. The method of claim 16 , further comprising successively resolving a preset or programmable number of most significant bit, MSB, values of the sampled analog input signal.
18. The method of claim 13 , wherein subtracting the dither occurs in an analog domain at an output of the DAC before a final SAR ADC output code is available.
19. The method of claim 13 , further comprising applying the dither signal with opposite polarity to an input of the DAC input, thereby subtracting a previously added dither value, D, from a present DAC output.
20. The method of claim 13 , further comprising adding and subsequently subtracting a random number value generated as the dither signal to the input of the DAC by a random number generator coupled to a summing junction located in the feedback path.