Dark current compensation for photon counting circuit

1. A circuit, comprising:

a charge sensitive amplifier (CSA) that includes an input to receive current from a photon sensor and generates an output signal that represents photons received by the sensor and dark current of the sensor; and

a control circuit to generate a compensation signal to offset the dark current from the photon sensor at the input of the CSA, wherein the control circuit couples feedback from the CSA to enable the compensation signal if the photon current received from the sensor is below a predetermined threshold and decouples the feedback from the CSA to disable the compensation signal if the photon current received from the sensor is above the predetermined threshold;

wherein the CSA includes a capacitor in a feedback path from the output of the CSA to the input to integrate the current received and the dark current from the photon sensor.

2. The circuit of claim 1 , further comprising a bias circuit to generate an input bias voltage at another input to the CSA and to generate an output bias voltage to an input of the control circuit to mitigate common mode voltage at the input of the CSA and at the input of the control circuit.

3. The circuit of claim 2 , wherein the input bias voltage and the output bias voltage are set to the same voltage level or set to different voltage levels via the bias circuit.

4. The circuit of claim 3 , wherein the bias circuit adjusts the output bias voltage at the input of the control circuit based on detected changes of dark current from the photon sensor, the detected changes based in part on detected changes in temperature of the photon sensor.

5. The circuit of claim 4 , the control circuit further comprising an amplifier and an adjustable current source, the amplifier amplifies the output signal from the CSA and drives the adjustable current source to offset the dark current at the input of the CSA based on the output signal from the CSA.

6. The circuit of claim 4 , further comprising a comparator that receives a predetermined trigger voltage from the bias circuit and generates an event signal indicating a photon has been received, the event signal disables the compensation signal during the portion of a time period that photons are detected by the CSA.

7. The circuit of claim 6 , further comprising a delay circuit to generate a reset signal from the event signal, the reset signal employed to discharge the capacitor in the feedback path.

8. A circuit, comprising:

a charge sensitive amplifier (CSA) that includes an input to receive current from a photon sensor and generates an output signal that represents photons received by the sensor and dark current of the sensor; and

a control circuit to generate a compensation signal to offset the dark current from the photon sensor at the input of the CSA, wherein the control circuit couples feedback from the CSA to enable the compensation signal if the photon current received from the sensor is below a predetermined threshold and decouples the feedback from the CSA to disable the compensation signal if the photon current received from the sensor is above the predetermined threshold;

further comprising a high pass filter at the output of the CSA to filter the output signal.

9. The circuit of claim 8 , further comprising an analog to digital converter (ADC) to record the photon energy level of the filtered output signal.

10. A circuit, comprising:

an integrator that includes an input to receive photon current and dark current from a photon sensor and generates an output signal that represents the photon current and the dark current received by the sensor at the input;

an amplifier to generate a compensation signal based on the output signal to offset dark current from the photon sensor at the input of the integrator; and

a switching circuit to couple the output signal to the input of the amplifier during a time period when received photon current is below a predetermined threshold and to decouple the output signal from the input of the amplifier during another time period when received photon current is above the predetermined threshold.

11. The circuit of claim 10 , wherein the integrator includes a feedback capacitor to integrate the photon current and the dark current from the photon sensor.

12. The circuit of claim 11 , further comprising a bias circuit to generate an input bias voltage at another input to the integrator and to generate an output bias voltage to an input of the amplifier to mitigate common mode voltage at the input of the integrator and at the input of the amplifier.

13. The circuit of claim 12 , wherein the input bias voltage and the output bias voltage are set to the same voltage level or set to different voltage levels via the bias circuit.

14. The circuit of claim 13 , wherein the bias circuit adjusts the output bias voltage at the input of the amplifier based on detected changes of dark current from the photon sensor, the detected changes based in part on detected changes in temperature of the photon sensor.

15. The circuit of claim 14 , further comprising an adjustable current source, wherein the amplifier drives the adjustable current source to offset the dark current at the input of the integrator based on the output signal from the integrator.

16. The circuit of claim 14 , further comprising a comparator that receives a predetermined trigger voltage from the bias circuit and generates an event signal indicating a photon has been received, the event signal disables the compensation signal during the portion of a time period that photons are detected by the integrator.

17. The circuit of claim 16 , further comprising a delay circuit to generate a reset signal from the event signal, the reset signal employed to discharge the feedback capacitor.

18. A method, comprising:

detecting a signal that represents photons from a photon sensor;

monitoring the signal to detect dark currents from the photon sensor;

generating a compensation signal to offset the dark currents from the sensor; and

disabling the updating of the compensation signal during a portion of a time period that photons are detected by the photon sensor;

further comprising:

generating an integrated signal from the signal that represents photons from the photon sensor; and

resetting the integrated signal after the portion of the time period that photons are detected by the photon sensor.