Demodulating a signal from intermittently illuminated region

An apparatus, comprising:

• a capacitor having an input configured to receive a time-varying input signal, the time-varying input signal being an amplified output of a photodiode configured to detect light from an intermittently illuminated region;
• a resistor having an input electrically connected to an output of the capacitor at a connection point, the resistor having an output configured to generate a time-varying output signal;
• and
• a switch configured to connect the connection point to ground during times when the region is not illuminated;
• wherein the time-varying output signal is a high-pass filtered version of the time-varying input signal during times when the region is illuminated, and a time-invariant ground signal during times when the region is not illuminated.

A system, comprising:

• a light emitting diode positioned to intermittently illuminate an intermittently illuminated region;
• a controller configured to turn on the light emitting diode by switching an illuminating trigger voltage from a first voltage less than a specified threshold voltage to a second voltage greater than the specified threshold voltage, the controller further configured to turn off the light emitting diode by switching the illuminating trigger voltage from the second voltage to the first voltage;
• a photodiode configured to detect light from the intermittently illuminated region;
• a transimpedance amplifier configured to amplify a photocurrent from the photodiode to produce a time-varying input signal;
• a demodulator configured to receive the illuminating trigger voltage and the time-varying input signal, and produce a time-varying output signal, wherein the time-varying output signal is a high-pass filtered version of the time-varying input signal during times when the region is illuminated, and a time-invariant ground signal during times when the region is not illuminated;
• and
• a secondary gain stage configured to amplify the time-varying output signal to produce an amplified time-varying output signal;
• wherein the controller is further configured to receive the amplified time-varying output signal.

A method, comprising:

• receiving a time-varying input signal, the time-varying input signal being an amplified output of a photodiode configured to detect light from an intermittently illuminated region;
• high-pass filtering the time-varying input signal during times when the region is illuminated;
• grounding the time-varying input signal during times when the region is not illuminated;
• and
• generating a time-varying output signal comprising the high-pass filtered version of the time-varying input signal during times when the region is illuminated, and a ground voltage during times when the region is not illuminated.