Non-linearity correction that is independent of input common mode, temperature variation, and process variation
In pipeline analog-to-digital converters (ADCs) the third harmonic can degrade the performance of the ADC, and conventional circuits that attempt to cancel this third harmonic are oftentimes sensitive to process variation, temperature variation, and common mode variations. Here a correction circuit is provided that includes a compensator that adjusts control voltages for MOS capacitors to generally ensures that the difference between the gate-source voltages and threshold voltages of MOS capacitors is generally maintained across variations of process, temperature, and common mode.
1. An apparatus comprising:
a sample-and-hold (S/H) circuit that receives an input signal; and
a correction circuit having:
a plurality of MOS capacitors that are each coupled to the S/H circuit and that each receives the input signal;
a compensator that receives a common mode voltage; and
a voltage divider that is coupled to the compensator so as to generate a plurality of control voltages relative to the common mode voltage, process variations of the MOS capacitors, and temperature variations of the NMOS capacitors and that is coupled to each of MOS capacitors so as to provide each MOS capacitor with at least one of a plurality of control voltages.
2. The apparatus of claim 1 , wherein the compensator further comprises:
an amplifier having a first input terminal, a second input terminal, and an output terminal, wherein the first input terminal of the amplifier receives the common mode voltage, and wherein the output terminal of the amplifier is coupled to the voltage divider;
a current source that is coupled to the second input terminal of the amplifier; and
a feedback circuit that is coupled between the output terminal of the amplifier and the second input terminal of the amplifier.
3. The apparatus of claim 2 , wherein the feedback circuit further comprises a diode and a resistor coupled in series with one another.
4. The apparatus of claim 3 , wherein the diode further comprises a diode-connected transistor.
5. The apparatus of claim 4 , wherein the diode-connected transistor is an NMOS transistor.
6. The apparatus of claim 5 , wherein the current source further comprises a first current source, and wherein the apparatus further comprises a second current source that is coupled to the voltage divider.
7. The apparatus of claim 6 , wherein the voltage divider further comprises a plurality of resistor coupled in series with one another.
8. An apparatus comprising:
a plurality of analog-to-digital converter (ADC) stages coupled in series with one another in a sequence, wherein the first ADC stage of the sequence includes:
a sample-and-hold (S/H) circuit that receives the analog input signal; and
a correction circuit having:
a plurality of MOS capacitors that are each coupled to the S/H circuit and that each receive the analog input signal;
a compensator that receives a common mode voltage; and
a voltage divider that is coupled to the compensator so as to generate a plurality of control voltages relative to the common mode voltage, process variations of the MOS capacitors, and temperature variations of the NMOS capacitors and that is coupled to each of MOS capacitors so as to provide each MOS capacitor with at least one of a plurality of control voltages; and
a digital output circuit that is coupled to each ADC stage and that outputs a digital output signal.
9. The apparatus of claim 8 , wherein the compensator further comprises:
an amplifier having a first input terminal, a second input terminal, and an output terminal, wherein the first input terminal of the amplifier receives the common mode voltage, and wherein the output terminal of the amplifier is coupled to the voltage divider;
a current source that is coupled to the second input terminal of the amplifier; and
a feedback circuit that is coupled between the output terminal of the amplifier and the second input terminal of the amplifier.
10. The apparatus of claim 9 , wherein the feedback circuit further comprises a diode and a resistor coupled in series with one another.
11. The apparatus of claim 10 , wherein the diode further comprises a diode-connected transistor.
12. The apparatus of claim 11 , wherein the diode-connected transistor is an NMOS transistor.
13. The apparatus of claim 12 , wherein the current source further comprises a first current source, and wherein the apparatus further comprises a second current source that is coupled to the voltage divider.
14. The apparatus of claim 13 , wherein the voltage divider further comprises a plurality of resistor coupled in series with one another.
15. The apparatus of claim 14 , wherein each ADC stage further comprises:
a sub-ADC that is coupled to an S/H circuit and that is coupled to the digital output circuit;
a digital-to-analog converter (DAC) that is coupled to the sub-ADC;
an adder that is coupled to the S/H circuit and the DAC so as to output a residue signal; and
a residue amplifier that is coupled to the adder.