ATLAS FSI project

The 32-channel front-end board for the optical alignment system of the ATLAS detector

This is an example of an electronics group case study taken from work we did for the Large Hadron Collider ATLAS detector project.

The optical alignment system allows absolute length measurements. It tracks deformations of the ATLAS Silicon Central Tracker. It does this by sending laser light into the ATLAS Silicon Central Tracker particle detector, channeling light from reflections of well placed minute mirrors into fibres which causes interference patterns which we can measure. These interference patterns contain distance information.

This daughterboard circuit receives the interference patterns of light and turns them into electrical signals. This uses Avalanche Photo Diodes (APDs). These devices make sure that small amounts of light still generate enough voltage to be able to measure the interference patterns.

The motherboard receives the electrical signals from the daughterboard. Each input channel is integrated before digitization. The integration time is adjustable between 250µs and 16000µs. The stored result at the end of the integration period is a voltage that represents the light intensity for that time slot.

Multiplexers select each stored channel, one after the other. This makes it possible to measure 32 channels with one ADC that samples at 330 kHz. The larger integrated circuits in the middle of the picture are the actual multiplexers taking the signals from the surrounding channels.

The selected channels are digitised. A programmable logic device (FPGA) generates the control signals that orchestrate the multiplexing and digitising.

To buffer the data, a memory daughterboard allows for 1 Gbit of local memory. Note that these Dynamic memory ICs (SDRAM) have their own Programmable Logic Device to manage them.

A 200 V bias is provided to the Avalanche Photo Diodes (APD) by a small DC to DC converter. It is given a value to optimise signal to noise ratio. This means neither too much nor too little voltage.

The complete system installed in the ATLAS cavern.