A high accuracy 22 bit sigma-delta converter for digital regulation of super-conducting magnet currents

The design of the latest CERN particle accelerator for high energy particle physics research, the Large Hadron Collider (LHC), has been underway for some years. The new machine requires stronger magnetic fields which necessitates the use of super-conducting magnets around most of the 27km ring tunnel. The powering of these magnets, with currents up to 13 kA, presents new challenges in precision, which can be summarised as an improvement in setting and regulation of certain currents to the part per million (ppm) level. This represents approximately a factor of ten improvement over present analogue methods. This paper will concentrate on the design and performance of the analogue to digital converter (ADC), which is the key element in the digital regulation loops employed in each magnet power converter