This is the front-end electronics for a homodyne quadrature interferometer device (see S J Cooper et al 2018 Class. Quantum Grav. 35 095007) used for sensing the motion of suspended test masses in gravitational wave experiments in order to keep them still.
The circuit is based on a design by David Hoyland at the University of Birmingham, but with some changes to make the circuit compatible with the standard power supply, digital input/output and components within the GEO collaboration. The board and front panel design is also entirely new.
The main functions of the board are to provide a stable, low noise voltage reference to act as a bias for the photodiodes, and to read out their photocurrent. The photocurrent is converted into a low output impedance voltage, filtered and (optionally) whitened before being provided as differential outputs capable of being sent across long distances to e.g. data acquisition hardware.
Digital inputs on the front panel provide the option to enable/disable whitening remotely, and digital outputs provide a readback mechanism for the relays that do the switching.
There is also a diagnostics connector on the board to assist debugging.
I can provide a test procedure document to anyone that wants it.