Maintaining consistent photodetector sensitivity in an optical measurement system
An illustrative optical measurement system includes a light source configured to emit light directed at a target. The optical measurement system further includes a photodetector configured to detect a photon of the light after the light is scattered by the target. The optical measurement system further includes a control circuit configured to receive a first input voltage that is a temperature-dependent voltage. The control circuit is further configured to receive a second input voltage that is a temperature-invariant voltage. The control circuit is further configured to output, based on a combination of the first input voltage and the second input voltage, a bias voltage for the photodetector, wherein the combination of the first and second input voltages is configured to cause the bias voltage to vary based on temperature.
1. An optical measurement system comprising:
a light source configured to emit light directed at a target;
a photodetector configured to detect a photon of the light after the light is scattered by the target; and
a control circuit configured to:
receive a first input voltage that is a temperature-dependent voltage;
receive a second input voltage that is a temperature-invariant voltage; and
output, based on a combination of the first input voltage and the second input voltage, a bias voltage for the photodetector, wherein the combination of the first and second input voltages is configured to cause the bias voltage to vary based on temperature.
2. The optical measurement system of claim 1 , wherein the combination of the first and second input voltages is based on a breakdown voltage of the photodetector such that the bias voltage is configured to vary based on the breakdown voltage.
3. The optical measurement system of claim 2 , wherein:
the breakdown voltage is a temperature-dependent voltage; and
the control circuit is configured to maintain the bias voltage a predetermined voltage level above the breakdown voltage.
4. The optical measurement system of claim 1 , wherein the control circuit comprises a charge pump configured to output the bias voltage.
5. The optical measurement system of claim 4 , wherein:
the first input voltage is a proportional-to-absolute temperature (PTAT) voltage;
the second input voltage is a bandgap voltage; and
the charge pump comprises a plurality of charge pump stages cascaded together including a first charge pump stage and a second charge pump stage,
the first charge pump stage configured to receive, as a clock voltage of the first charge pump stage, the PTAT voltage, and
the second charge pump stage configured to receive, as a clock voltage of the second charge pump stage, the bandgap voltage.
6. The optical measurement system of claim 1 further comprising an array of photodetectors including the photodetector, and
wherein the control circuit configured to output the bias voltage for the photodetector includes outputting the bias voltage for the array of photodetectors.
7. The optical measurement system of claim 1 , wherein the photodetector comprises:
a single photon avalanche diode (SPAD); and
a fast gating circuit configured to arm and disarm the SPAD.
8. The optical measurement system of claim 1 , wherein the photodetector is included in a wearable device configured to be worn by a user.
9. The optical measurement system of claim 8 , wherein the wearable device includes a head-mountable component configured to be worn on a head of a user.
10. A method comprising:
receiving, by a control circuit, a first input voltage that is a temperature-dependent voltage;
receiving, by the control circuit, a second input voltage that is a temperature-invariant voltage; and
outputting, by the control circuit and based on a combination of the first input voltage and the second input voltage, a bias voltage for a photodetector, wherein the combination of the first and second input voltages is configured to cause the bias voltage to vary based on temperature.
11. The method of claim 10 , wherein the combination of the first and second input voltages is based on a breakdown voltage of the photodetector such that the bias voltage is configured to vary based on the breakdown voltage.
12. The method of claim 11 , wherein:
the breakdown voltage is a temperature-dependent voltage; and
the outputting the bias voltage comprises maintaining, by the control circuit, the bias voltage at a predetermined voltage level above the breakdown voltage.
13. The method of claim 10 , wherein:
the control circuit comprises a charge pump; and
the outputting the bias voltage comprises providing an output of the charge pump.
14. The method of claim 13 , wherein:
the first input voltage is a proportional-to-absolute temperature (PTAT) voltage;
the second input voltage is a bandgap voltage;
the charge pump comprises a plurality of charge pump stages cascaded together including a first charge pump stage and a second charge pump stage;
the receiving the first input voltage by the control circuit comprises the first charge pump stage receiving, as a clock voltage input of the first charge pump stage, the PTAT voltage, and
the receiving the second input voltage by the control circuit comprises the second charge pump stage receiving, as a clock voltage input of the second charge pump stage, the bandgap voltage.
15. The method of claim 13 , wherein the outputting the bias voltage for the photodetector comprises outputting the bias voltage for an array of photodetectors including the photodetector.