Active load clamp for a track-and-hold circuit

1. A method of tracking and holding a voltage, such method comprising the steps of:

providing an external analog voltage to an input of a track and hold circuit during a tracking mode of the track and hold circuit;

storing a representation of the sampled external voltage in a storage device of the track and hold circuit during the tracking mode;

blocking a signal path between the external voltage and the storage device during a holding mode of the track and hold circuit; and

clamping the input of the track and hold circuit to a predetermined voltage during the holding mode.

2. The method of tracking and holding a voltage as in claim 1 wherein the step of storing the representation further comprises coupling the input signal to the storage device through an emitter-follower transistor.

3. The method of tracking and holding a voltage as in claim 2 further comprising coupling a collector of the emitter-follower transistor to a supply voltage and an emitter of the emitter-follower transistor to the storage device.

4. The method of tracking and holding a voltage as in claim 3 further comprising coupling a tracking current sink in parallel with the storage device.

5. The method of tracking and holding a voltage as in claim 4 further comprising defining the storage device as a capacitor.

6. The method of tracking and holding a voltage as in claim 5 wherein the step of blocking further comprises disconnecting the tracking current sink from its parallel connection with the storage device.

7. The method of tracking and holding a voltage as in claim 6 wherein the step of clamping further comprises reconnecting the tracking current sink to a base of the emitter-follower transistor during the holding mode.

8. The method of tracking and holding a voltage as in claim 7 wherein the step of clamping further comprises coupling a helper current sink to the base of the emitter-follower circuit.

9. The method of tracking and holding a voltage as in claim 8 wherein the step of clamping the input to the predetermined voltage further comprises coupling a filtering device between a voltage reference and the input.

10. The method of tracking and holding a voltage as in claim 9 wherein the step of coupling the filtering device between the voltage reference and the input further comprises coupling a diode between a first voltage reference and the input.

11. The method of tracking and holding a voltage as in claim 10 wherein the step of coupling the filtering device between the voltage reference and the input further comprises coupling a resistor coupled between the input and a second voltage reference.

12. An apparatus for tracking and holding a voltage, such apparatus comprising:

means for providing an external analog voltage to an input of a track and hold circuit during a tracking mode of the track and hold circuit;

means for storing a representation of the sampled external voltage in a storage device of the track and hold circuit during the tracking mode;

means for blocking a signal path between the external voltage and the storage device during a holding mode of the track and hold circuit; and

means for clamping the input of the track and hold circuit to a predetermined voltage during the holding mode.

13. The apparatus for tracking and holding a voltage as in claim 12 wherein the means for storing the representation further comprises means for coupling the input signal to the storage device through an emitter-follower transistor.

14. The apparatus for tracking and holding a voltage as in claim 13 further comprising means for coupling a collector of the emitter-follower transistor to a supply voltage and an emitter of the emitter-follower transistor to the storage device.

15. The apparatus for tracking and holding a voltage as in claim 14 further comprising means for coupling a tracking current sink in parallel with the storage device.

16. The apparatus for tracking and holding a voltage as in claim 15 further comprising defining the storage device as a capacitor.

17. The apparatus for tracking and holding a voltage as in claim 16 wherein the means for blocking further comprises means for disconnecting the tracking current sink from the storage device.

18. The apparatus for tracking and holding a voltage as in claim 17 wherein the means for clamping further comprises means for connecting the tracking current sink to a base of the emitter-follower transistor during the holding mode.

19. The apparatus for tracking and holding a voltage as in claim 18 wherein the means for clamping further comprises means for coupling a helper current sink to the base of the emitter-follower circuit.

20. The apparatus for tracking and holding a voltage as in claim 19 wherein the means for clamping the input to the predetermined voltage further comprises means for coupling a filtering device between a voltage reference and the input.

21. The apparatus for tracking and holding a voltage as in claim 20 wherein the means for coupling the filtering device further comprises a diode coupled between a first voltage reference and the input.

22. The apparatus for tracking and holding a voltage as in claim 21 wherein the means for coupling the filtering devices further comprises a resistor coupled between the input and a second voltage reference.

23. A track and hold circuit for tracking and holding a voltage, such apparatus comprising:

an external analog voltage coupled to an input of the track and hold circuit during a tracking mode of the track and hold circuit;

a storage device adapted to store a representation of the external analog voltage during the tracking mode;

a blocking circuit that blocks a signal path between the external voltage and the storage device during a holding mode of the track and hold circuit; and

a clamping circuit that clamps the input of the track and hold circuit to a predetermined voltage during the holding mode.

24. The apparatus for tracking and holding a voltage as in claim 23 wherein the means for storing the representation further comprises a first emitter-follower transistor that couples the input signal to the storage device.

25. The apparatus for tracking and holding a voltage as in claim 24 further comprising a collector of the first emitter-follower transistor coupled to a supply voltage and an emitter of the first emitter-follower transistor to the storage device.

26. The apparatus for tracking and holding a voltage as in claim 25 further comprising a tracking current sink coupled in parallel with the storage device.

27. The apparatus for tracking and holding a voltage as in claim 26 further comprising defining the storage device as a capacitor.

28. The apparatus for tracking and holding a voltage as in claim 27 wherein the blocking circuit further comprises a second transistor that open circuits the tracking current sink.

29. The apparatus for tracking and holding a voltage as in claim 28 further comprising a third transistor that connects the tracking current sink to a base of the emitter-follower circuit during the holding mode.

30. The apparatus for tracking and holding a voltage as in claim 29 further comprises a helper current sink that is coupled to the base of the emitter-follower circuit during the holding mode.

31. The apparatus for tracking and holding a voltage as in claim 30 further comprising a filtering device coupled between a voltage reference and the input to the track and hold circuit.

32. The apparatus for tracking and holding a voltage as in claim 31 wherein the filtering device further comprises a diode coupled between a first voltage reference and the input.

33. The apparatus for tracking and holding a voltage as in claim 32 wherein the filtering devices further comprises a resistor coupled between the input and a second voltage reference.