Maintaining consistent photodetector sensitivity in an optical measurement system
An exemplary 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. A wearable system for use by a user comprising:
a head-mountable component configured to be attached to a head of the user, the head-mountable component comprising a photodetector configured to detect a photon from a light pulse after the light pulse reflects off a target within the head; 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 wearable 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 wearable 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 wearable system of claim 1 , wherein the control circuit comprises a charge pump configured to output the bias voltage.
5. The wearable 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 input of the first charge pump stage, the PTAT voltage, and
the second charge pump stage configured to receive, as a clock voltage input of the second charge pump stage, the bandgap voltage.
6. The wearable system of claim 1 , wherein:
the head-mountable component further comprises 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 wearable 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.