A system comprising: a light emitter (121, 260, 720), wherein the light emitter is configured to illuminate a portion of subsurface vasculature via an external body surface; a photodetector (125, 250, 710a), wherein the photodetector is configured to receive light emitted from the portion of subsurface vasculature responsive to illumination by the light emitter; an electrocardiogram sensor (135, 270, 710b), wherein the electrocardiogram sensor is configured to detect an electrocardiographic signal via the external body surface; an amplifier (230, 713); a multiplexer (220, 711), wherein the multiplexer is electrically connected to an output of the photodetector, an output of the electrocardiogram sensor, and an input of the amplifier, and wherein the multiplexer is controllable to selectively connect the output of the photodetector and the output of the electrocardiogram sensor to the input of the amplifier; an analog-to-digital converter, wherein the analog-to-digital converter is configured to generate digital codes based on an output of the amplifier; and a controller (140, 210, 750), wherein the controller is configured to perform controller operations comprising: operating the light emitter, during a first plurality of specified periods of time, to illuminate the portion of subsurface vasculature; operating the multiplexer, during the first plurality of specified periods of time, to connect the output of the photodetector to the input of the amplifier; operating the analog-to-digital converter to generate a first plurality of digital codes based on the output of the amplifier during the first plurality of specified periods of time; operating the multiplexer, during a second plurality of specified periods of time, to connect the output of the electrocardiogram sensor to the input of the amplifier; and operating the analog-to-digital converter to generate a second plurality of digital codes based on the output of the amplifier during the second plurality of specified periods of time. a light emitter (121, 260, 720), wherein the light emitter is configured to illuminate a portion of subsurface vasculature via an external body surface; a photodetector (125, 250, 710a), wherein the photodetector is configured to receive light emitted from the portion of subsurface vasculature responsive to illumination by the light emitter; an electrocardiogram sensor (135, 270, 710b), wherein the electrocardiogram sensor is configured to detect an electrocardiographic signal via the external body surface; an amplifier (230, 713); a multiplexer (220, 711), wherein the multiplexer is electrically connected to an output of the photodetector, an output of the electrocardiogram sensor, and an input of the amplifier, and wherein the multiplexer is controllable to selectively connect the output of the photodetector and the output of the electrocardiogram sensor to the input of the amplifier; an analog-to-digital converter, wherein the analog-to-digital converter is configured to generate digital codes based on an output of the amplifier; and a controller (140, 210, 750), wherein the controller is configured to perform controller operations comprising: operating the light emitter, during a first plurality of specified periods of time, to illuminate the portion of subsurface vasculature; operating the multiplexer, during the first plurality of specified periods of time, to connect the output of the photodetector to the input of the amplifier; operating the analog-to-digital converter to generate a first plurality of digital codes based on the output of the amplifier during the first plurality of specified periods of time; operating the multiplexer, during a second plurality of specified periods of time, to connect the output of the electrocardiogram sensor to the input of the amplifier; and operating the analog-to-digital converter to generate a second plurality of digital codes based on the output of the amplifier during the second plurality of specified periods of time. operating the light emitter, during a first plurality of specified periods of time, to illuminate the portion of subsurface vasculature; operating the multiplexer, during the first plurality of specified periods of time, to connect the output of the photodetector to the input of the amplifier; operating the analog-to-digital converter to generate a first plurality of digital codes based on the output of the amplifier during the first plurality of specified periods of time; operating the multiplexer, during a second plurality of specified periods of time, to connect the output of the electrocardiogram sensor to the input of the amplifier; and operating the analog-to-digital converter to generate a second plurality of digital codes based on the output of the amplifier during the second plurality of specified periods of time.