DIGITAL-CONTROL OF A DIFFERENTIAL SCANNING MICROCALORIMETER

A detailed description of the digital control for a differential scann ing microcalorimeter to be used in the study of biomolecules is given. Three control loops and an adequate control program ensure that the i nstrument works under adiabatic conditions and the temperature differe nce between the sample and reference cells is as close as possible to zero. A static and dynamic characterization of the instrument is also described. The gain of the control loop can be selected by the operato r, thus changing the calibration constant, signal/noise ratio (SNR) an d time constant of the instrument according to experimental requiremen ts. Some characteristics of the instrument are: (a) any scan rate betw een 0.1 and 2 K min(-1), with deviations from linearity and reproducib ility about 0.2-0.3%, can be selected; (b) its calibration constants a t each scan rate are highly reproducible (about 0.4%); (c) the reprodu cibility of the instrument upon refilling of the cells is about 10 mu J K-1; (d) the noise in the sensor is about 2 nV; and (e) the time con stant of the instrument changes from 7 to 70 s depending on the contro l loop gain used.