A 1nA-Photocurrent Optical Receiver used in Fire and Smoke Detection Systems

A receiver circuit being able to detect very small currents from an external photodiode and convert them into a voltage treatable by an A/D converter is presented. Its equivalent input noise current is 34 pArms at a system bandwidth of approx. 40kHz and equivalent DC offset is less than InA. The circuit has been designed in a standard low-cost `digital´ 2¿m double-metal (single-poly) medium-voltage CMOS process. The receiver system is briefly explained and the most relevant measurement data are discussed, this with special emphasis to reproduceability in fabrication. Comparisons are made between a differential and a single-ended version. The active chip surface occupied by the receiver block itself is 5.3 mm² and its current consumption is less than 500¿A @ 5V.