Low-noise sampling system for photo current detection with monolithically integrated photo diodes

A light detection system for infrared light signals using monolithically integrated photo diodes is presented. Each channel consists of a low-noise input amplifier, a low-pass filter and a 10-bit tracking analog-to-digital converter (ADC). Maximum light signal level which can be handled is 1 µW of optical power on each photo diode, consisting of an dc part and an ac part. Ratio of the dc to the ac part may vary up to 1000. The frequency range of the ac part can vary in the range of 10Hz to 50kHz. Output signal is available as an 8-bit word for the dc part and a 10-bit word for the ac part. Driver circuitry for driving a laser diode is also integrated on the silicon, the laser diode can be driven by a current up to 11mA, whilst the noise of that current is below only 0.5µA. 10-bit sampling of all six channels up to 3Msamples / sec. is possible by an overall current consumption (without laser current) of the chip of only 4mA. The system was realised in a 0.5µm standard BiCMOS technology.