Shown here is a block diagram of the Si1120. The analog front end consists of two photodiodes that measure the incoming electromagnetic energy. One of the detectors is used to measure the IR energy and the other is used to measure the energy in the visible spectrum. The complete analog signal chain is integrated and provides the low going PRX output which is a pulse proportional to the light intensity. The TXO output is a constant current sink that can be set to 50mA or 400mA (with no current limiting) in order to directly drive an IR LED for the emitter. STX, SC and MD are control inputs to set the operating mode and STX also starts the conversion process.