Light irradiation is used in the photo-diode to generate the current, termed as IPD. As the received radiation power is proportional to the sensor signal, a very low current arises. The sensor current can be electronically evaluated by amplifying and converting it into an electrical voltage.
The evaluation of the current can be done precisely by using a negligible input impedance of the amplifier. A trans-impedance amplifier is used to achieve the circuitry where the output voltage VPD is fed back via a resistor to the input.
The dynamic range of the amplifier is increased by the common-mode voltage (VCM: Common-Mode Voltage) where the positive input of the amplifiers can be adjusted and implemented with an operational amplifier. An operational amplifier is with a negligibly small input current. As per the principle, the current generated in the resistor is compensated by the input current.
The photosensor which detects the signals is characterized as very short pulses with steep edges. The generated current is the ambient lighting or known as the diode’s dark current.
The changing currents are slowly relative as they can largely eliminate the photodiode that is coupled with a capacitor. A short circuit is represented with a high dynamic signal where the capacitor is changed so that the currents are practical of the same size.
Source:- Trends in der Automobil-Sensorik