Signal Conditioning for Cryogenic Thermometry in the LHC

Temperature measurement is a key issue in the LHC, as it will be used to regulate the cooling of the superconducting magnets. The compromise between available cooling power and the coil superconducting characteristics leads to a restricted temperature control band, around 1.9 K. An absolute accuracy DeltaT < 10 mK below 2.2 K, and DeltaT < 5 K above 25 K, is necessary. For resistive thermometers covering the full temperature range, and having a negative dR/dT sensitivity, this is typically equivalent to a relative accuracy DeltaR/R of 3 10**-3 over 3 resistance decades. Also, to limit the thermometer's self-heating, the sensing current must be limited to few muA. Furthermore, the radiation levels next to the accelerator are expected to degrade significantly the performance of conventional analogue electronics. As these stringent requirements are not met by commercial conditioners, three different architectures have been developed at CERN. The first compresses the input dynamic range using a logarithmic transfer function; the second partitions the input range into three linear regions; the third converts resistance linearly into the frequency of a square wave. They fulfil the above specifications and provide industrial robustness in terms of thermal drift, galvanic protection, and compact packaging, while optimising cost-to-performance ratio. This paper describes the principles of their design, compares their characteristics and shows results of field tests. Future developmens include ASIC versions, Fieldbus interfacing, and radiation tolerant re-design.