In this paper we present the results of the first tests performed at MPIA on the detector system of MIDI, the Mid-IR interferometric instrument for VLTI. Interferometric observations at 10 micrometers , while having advantages with respect to near-IR and optical wavelengths in terms of seeing and coherence time, must face the problem of the strong thermal background coming from the telescope and from the sky. In order to reach background limited performances in the observing conditions foreseen for MIDI at Paranal, the detector and the associated read-out electronics must comply to strict requirements. The different MIDI observing modes are characterized by widely different values of background per pixel; in high background conditions the main problem is to avoid saturation, while in medium-background conditions we are close to read-out noise limited conditions. Therefore, large integration capacity and high speed must be reached with low read-out noise. The read-out electronic chain is described; at maximum speed, one full 16-bit 320 X 240 pixels2 frame can be read in 3.6 msec. Since only a portion of the detector's field of view will contain useful information, only selected parts of the detector will be read, thus increasing the frame rate. We briefly review the different read-out strategies adopted; the correspondent operations on the detector are described. We also present the results of the first tests performed at MPIA on the whole detector subsystem, using a bare multiplexer of the selected detector (a 320 X 240 Si:As IBC array from Raytheon Corp.).