DIRT INSENSITIVE OPTICAL PAPER PATH SENSOR

摘要

The present invention is concerned with a self-adjusting document sensor compensating for degradation of the sensor system. A suitable light source and a detector are provided, the output of the detector being fed into an amplifier whose gain depends upon a feedback signal. Periodically, the output of the amplifier is compared to a reference. If the output of the amplifier falls below the reference, a pulse is sent to a ripple counter whose digital output is fed back to the amplifier to change the gain of the amplifier.If the detector is an unbiased photodiode operating in the transconductance mode, the leakage currents and their subsequent effect on output with amplifier gain changes will be minimized.