Sensor Scheme with Ambient Light Correction

In studying estimation methods for control systems I once had a professor say "use all available information". This may seem trivial but it has served as very good advice over many years. In a recent system I have been designing a sensor array to detect light from a display for purposes of calibrating pixel timing versus RBG colors. This method is sensitive to ambient light corrupting the SNR of the signal. However it was observed that the measurement scheme is taking readings in the KHz time span but most ambient light is DC.

As an aside even a florescent bulb is primarily DC ambient luminance. The 60Hz AC mains is applied across a phosphor tube exciting the phosphor to emit light. But due to the persistence the luminance is primarily DC.

The scheme used for correction was to detect when the measurements are not being made. This was readily done by monitoring the control signal to an ADC. When the measurements have been idle for a period defined by a one-shot delay an analog multiplexer circuit integrates the output signal and applies a DC bias to the sensor transimpedance amplifier input. When the output is at zero DC the integrator stops moving and the output stays at zero volts. While a measurement is being made this circuit is opened and the integrator value is held during the measurement time. The result is a sample & hold feedback circuit that dynamically corrects for DC ambient light and prevents saturation of the input stages.