We present in this paper the results of laboratory tests on the detector system for PYRAMIR, the infrared wavefront sensor for ALFA, the Adaptive Optics system at Calar Alto Observatory. PYRAMIR will use, at least in a first phase, a Hawaii-I detector, with 4 512x512 pixels2 quadrants which are read-out in parallel on 4 independent output channels. Since wavefront sensing in the infrared requires high frame rates and since the signal of the pyramid
wavefront sensor is distributed on a small fraction of the detector area, the detector is operated in a windowed mode. Setting the
pixel clock to the fastest speed supported by the chip without a significant increase in read noise and by addressing a 64x64 window, for instance, we are able to reach frame rates in excess of 150 Hz. We show our measurements of total read noise obtained at this relatively high read-out speed, as well as the results of our tests concerning linearity and sensitivity. The results show that
the noise introduced by the read-out electronics itself is negligible compared with the intrinsic read-noise of the detector.
In order to maximize the read-out efficiency we use differential measurements on a sequence of non destructive read-outs. We discuss the main characteristics of the detector when operating in this mode.