Method for stabilizing the gain of a discrete-state automatic gain control circuit

1. An automatic gain controller (AGC), comprising:

an amplifier including a variable gain;

a second order low-pass filter including an input coupled to an output of the amplifier;

a threshold detection circuit including an input coupled to an output of the second order low-pass filter to compare an output signal of the second order low-pass filter to a threshold and an output of the threshold detection circuit coupled to control the variable gain of the amplifier; and

a state machine coupled between the output of the threshold detection circuit and the amplifier, wherein the state machine is configured to transition based on a current state of the state machine.

2. The AGC of claim 1 , wherein the second order low-pass filter further includes a Sallen-Key topology.

3. The AGC of claim 2 , wherein the Sallen-Key topology comprises a quality factor between 1.4 and 1.6.

4. The AGC of claim 1 , further including a photodiode coupled to an input of the amplifier.

5. An automatic gain controller (AGC), comprising:

an amplifier including a variable gain;

a resonant low-pass filter coupled to an output of the amplifier;

a resistor coupled between the output of the amplifier and an input of the amplifier; and

a threshold detection circuit including an input coupled to an output of the resonant low-pass filter and a binary output coupled to control the variable gain of the amplifier.

6. The AGC of claim 5 , wherein the resonant low-pass filter includes a Sallen-Key topology.

7. The AGC of claim 6 , wherein the Sallen-Key topology includes a quality factor between 1.4 and 1.6.

8. The AGC of claim 5 , further including a photodiode coupled to an input of the amplifier.

9. The AGC of claim 5 , further including a differential amplifier coupled to the output of the amplifier.

10. The AGC of claim 5 , further including a state machine coupled between the threshold detection circuit and amplifier.

11. An automatic gain controller (AGC), comprising:

an amplifier;

a resonant low-pass filter coupled to the amplifier, wherein the resonant low-pass filter includes a Sallen-Key topology; and

a threshold comparator coupled to an output of the low-pass filter and configured to control a gain of the amplifier, wherein the threshold comparator includes hysteresis.

12. The AGC of claim 11 , wherein the Sallen-Key topology includes a quality factor between 1.4 and 1.6.

13. The AGC of claim 11 , further including a state machine coupled between the threshold comparator and resonant low-pass filter.

14. The AGC of claim 11 , further including a photodiode coupled to an input of the amplifier.

15. The AGC of claim 11 , further including a differential amplifier coupled to an output of the amplifier.

16. A method of making an automatic gain controller (AGC), comprising:

providing an amplifier;

coupling a resonant low-pass filter to an output of the amplifier; and

providing a threshold detection circuit coupled to an output of the resonant low-pass filter and configured to control a gain of the amplifier, wherein the threshold detection circuit includes hysteresis and the resonant low-pass filter is configured to exhibit overshoot to trigger the hysteresis of the threshold detection circuit.

17. The method of claim 16 , further including providing the resonant low-pass filter comprising a Sallen-Key topology.

18. The method of claim 17 , further including providing the Sallen-Key topology comprising a quality factor between 1.4 and 1.6.

19. The method of claim 16 , further including a resistor coupled between an input of the amplifier and an output of the amplifier.

20. The method of claim 16 , further including a state machine coupled between the threshold detection circuit and amplifier.

21. The AGC of claim 1 , wherein the variable gain of the amplifier includes a plurality of discrete gain levels.

22. The AGC of claim 5 , wherein the variable gain of the amplifier includes a plurality of discrete gain levels.