Method and apparatus for an active negative-capacitor circuit to cancel the input capacitance of comparators
A circuit comprises a first amplifier coupled to a first and a second node; a differential capacitive load coupled to the first and the second node, the differential capacitive load coupled between drains of transistors in a cross coupled transistor circuit; a current mirror coupled to a source of each transistor; and a capacitor coupled between the sources of the transistors. A plurality of amplifiers can be coupled to the differential capacitive load, wherein each amplifier comprises a clock-less pre-amplifier of a comparator. The amplifiers may be abutted to one another such that an active transistor of a first differential stage in a first amplifier behaves as a dummy transistor for an adjacent differential stage in a second amplifier.
1. A method, comprising:
coupling a differential output of a differential amplifier to a differential interconnect, the differential interconnect including a first node and a second node;
coupling a differential flash analog-to-digital convertor, having a differential input, to the differential interconnect; and
coupling a negative-capacitance circuit to the differential interconnect.
2. The method of claim 1 , further comprising:
coupling the differential interconnect to a plurality of comparators.
3. The method of claim 1 , wherein the negative-capacitance circuit comprises:
employing a first transistor to couple the first node to a first current node;
employing a second transistor to couple the second node to a second current node; and
employing a capacitor to cross-couple a source of the first transistor to a source of the second transistor.
4. The method of claim 3 , further comprising:
employing a third transistor to couple the first current node to a supply voltage; and
employing a fourth transistor to couple the second current node to the supply voltage.
5. The method of claim 4 , wherein the third transistor is matched to the fourth transistor.
6. The method of claim 5 , wherein a current in the third transistor substantially equals a current in the fourth transistor.
7. The method of claim 1 , wherein the differential amplifier is a programmable gain amplifier.
8. An apparatus comprising:
a differential interconnect including a first node and a second node;
a differential amplifier configured to provide a differential analog signal, the differential amplifier having a differential output that is coupled to the differential interconnect;
a differential flash analog-to-digital convertor having a differential input that is coupled to the differential interconnect; and
a negative-capacitance circuit coupled to the differential interconnect.
9. The apparatus of claim 8 , further comprising a plurality of comparators coupled to the differential interconnect.
10. The apparatus of claim 8 , wherein the negative-capacitance circuit comprises:
a first transistor coupling the first node to a first current node;
a second transistor coupling the second node to a second current node; and
a capacitor cross-coupling a source of the first transistor to a source of the second transistor.
11. The apparatus of claim 10 , wherein the differential amplifier is a programmable gain amplifier.
12. The apparatus of claim 10 , further comprising:
a third transistor coupling the first current node to a voltage supply; and
a fourth transistor coupling the second current node to the supply voltage.
13. The apparatus of claim 12 , further comprising:
a control circuit coupled to a gate of the third transistor and a gate of the fourth transistor, wherein the control circuit is configured to control a current in each of the third and fourth transistors.
14. The apparatus of claim 13 , wherein the control circuit is at least one of analog controlled and digitally controlled.
15. The apparatus of claim 13 , wherein the current in the third transistor substantially equals to the current in the fourth transistor.
16. The apparatus of claim 12 , wherein the third transistor is matched to the fourth transistor.
17. The method of claim 4 , further comprising:
coupling a control circuit to a gate of the third transistor and a gate of the fourth transistor, wherein the control circuit is configured to control a current in each of the third and fourth transistors.
18. The method of claim 17 , further comprising:
employing a fifth transistor to couple the control circuit to the gates of the third and fourth transistors.
19. The method of claim 17 , wherein the control circuit is analog controlled and/or digitally controlled.
20. The apparatus of claim 13 , further comprising:
a fifth transistor coupling the control circuit to the gates of the third and fourth transistors.