Dynamic tracking nonlinearity correction
View Patent ↗An analog-to-digital converter (ADC) is used for dynamic tracking nonlinearity correction. The correction employs an analog sampling technique to determine the signal derivative by measuring the derivative current arising from sampling an analog input signal undergoing analog-to-digital conversion, at the sampling instant. The analog derivative sampling technique achieves significant reduction in power consumption with less complexity compared with a digital approach, with strong improvements in HD 3 , SDFR, and IM 3 measures.
1. A system comprising:
analog-to-digital sample-and-hold circuitry comprising:
a bottom plate transistor characterized by a non-linear resistance; and
a top plate transistor coupled to a sampling capacitor;
a late clock input coupled to the bottom plate transistor and operable to carry a late clock;
an early clock input coupled to the top plate transistor, where the early clock input is operable to carry an early clock, with respect to the late clock; and
an analog-to-digital converter (ADC) following the sampling capacitor, the ADC configured to:
convert a derivative current flowing through the non-linear resistance and the sampling capacitor to a digital representation; and
provide the digital representation on a sampling output.
2. The system of claim 1 , where:
the ADC comprises a converter clock input coupled to the early clock input.
3. The system of claim 1 , further comprising:
current-to-voltage conversion circuitry between the sampling capacitor and the ADC, the current-to-voltage conversion circuitry configured to convert the derivative current to a derivative voltage.
4. The system of claim 3 , further comprising:
level-shifting circuitry between the current-to-voltage conversion circuitry and the ADC, the level-shifting circuitry configured to adjust the derivative voltage for input compatibility with the ADC.
5. The system of claim 1 , where:
the ADC comprises a differential input ADC; and
the analog-to-digital sample-and-hold circuitry is replicated in differential sampling branches comprising differential derivative current sampling outputs to the differential input ADC.
6. The system of claim 1 , further comprising:
error correction circuitry coupled to the sampling output, where the error correction circuitry comprises an error calculation circuit and an adder circuit.
7. A system comprising:
an input buffer operable to receive an analog input signal;
a main-path analog-to-digital converter (ADC) coupled to the input buffer and operable to convert the analog input signal to a digital signal representation;
a derivative sampling circuit coupled to the input buffer, the derivative sampling circuit comprising:
analog-to-digital sample-and-hold circuitry comprising:
a bottom plate switch comprising a non-linear resistance;
a sampling capacitor; and
a top plate switch in series with the sampling capacitor;
a current sampling analog-to-digital converter (ADC) following the sampling capacitor, the current sampling ADC configured to:
convert a derivative current flowing through the non-linear resistance and the sampling capacitor to a digital representation; and
provide the digital representation on a sampling output to error correction circuitry;
a late clock input coupled to the bottom plate switch and operable to carry a late clock;
an early clock input coupled to the top plate switch, where the early clock input is operable to carry an early clock that is active before the late clock; and
where the error correction circuitry comprises:
an error calculation circuit;
an adder circuit coupled to a main-path ADC and the error calculation circuit; and
a least mean square processor coupled to the error calculation circuit.
8. The system of claim 7 , further comprising:
current-to-voltage conversion circuitry between the sampling capacitor and the current sampling ADC, the current-to-voltage conversion circuitry configured to convert the derivative current to a derivative voltage;
level-shifting circuitry between the current-to-voltage conversion circuitry and the current sampling ADC, the level-shifting circuitry configured to adjust the derivative voltage for input compatibility with the current sampling ADC; and
where:
the current sampling ADC comprises a differential input ADC; and
the analog-to-digital sample-and-hold circuitry, the current-to-voltage conversion circuitry, and the level-shifting circuitry are replicated in differential sampling branches comprising differential derivative current sampling outputs to the differential input ADC.
9. A method comprising:
sampling an analog input signal with analog-to-digital sample-and-hold circuitry comprising:
a bottom plate transistor characterized by a non-linear resistance; and
a top plate transistor in series with a sampling capacitor;
late clocking the bottom plate transistor;
early clocking the top plate transistor; and
measuring derivative current with an analog-to-digital converter (ADC) following the sampling capacitor, by:
converting a derivative current flowing through the non-linear resistance and the sampling capacitor to a digital representation; and
providing the digital representation on a sampling output to error correction circuitry.
10. The method of claim 9 , further comprising:
generating an early clock as an advance version of a late clock to carry out the late clocking and the early clocking.
11. The method of claim 10 , further comprising:
clocking the ADC with the early clock.
12. The method of claim 9 , further comprising:
performing current-to-voltage conversion between the sampling capacitor and the ADC, the current-to-voltage conversion converting the derivative current to a derivative voltage.
13. The method of claim 12 , further comprising:
performing level-shifting between the current-to-voltage conversion circuitry and the ADC, the level-shifting adjusting the derivative voltage for input compatibility with the ADC.
14. The method of claim 9 , where:
the ADC comprises a differential input ADC; and
the analog-to-digital sample-and-hold circuitry is replicated in differential sampling branches comprising differential derivative current sampling outputs to the differential input ADC.
15. The method of claim 9 , further comprising:
processing the sampling output with error correction circuitry; and
correcting a main-path ADC output responsive to the processing.
16. The method of claim 15 , where correcting the main-path ADC output comprises generating a calibrated sample output.
17. The method of claim 16 , further comprising determining, after correcting the main-path ADC output, a filter coefficient for the error correction circuitry that produces a minimum error between the calibrated sample output and the analog input signal.
18. The method of claim 17 , where determining the filter coefficient comprises processing the calibrated sample output and the analog input signal with a least mean square processor.
19. The system of claim 6 , where the adder circuit comprises:
a first input coupled to the main-path ADC output; and
a second input coupled to the sampling output;
a calibrated sample output output coupled to the first and second inputs.
20. The system of claim 19 , further comprising a least mean square processor configured to:
determine a filter coefficient for the error correction circuitry by comparing the calibrated sample output to an analog input signal used to generate the derivative current.