A USB high resolution lock-in photometer

A design is presented, in which the DDC112 current-input analogue-to-digital converter is combined with a PIC microcontroller and associated circuitry to give a simple and economical dual-beam photometer / electrometer. The PIC supervises the operation of the analogue-digital converter and performs a lock-in function using a rolling average filter, which enables intensity measurement to parts per million. It also synchronously controls a regulated current source to drive, typically, one or more power LEDs as light sources. It has been applied in various fluorescence and absorbance measurements and can be used with Cavity Enhanced Absorption Spectrometry to determine ultra-low absorbance. An expanded version has been created with a PIC32 processor handling eight channels for a PixelSensor multispectral detector. All circuit details and source code are made available.