The ANTARES collaboration has undertaken to build a deep-sea neutrino telescope. The technique consists in deploying an array of photomultipliers arranged in a lattice near the sea bed at 2400 m depth. The array detects Cherenkov light from muons produced by neutrino interactions in the sea water and the rock beneath. The neutrino direction and energy are obtained from the time of arrival of the Cherenkov light and from the light yield. These measurements are performed by the Analogue Ring Sampler, an Application Specific Integrated Circuit designed to read and digitise the photomultiplier signal. The chip has been implemented in the front-end electronics of ANTARES. This paper presents measurements of the chip performances in this environment.