1. A readout circuit for an X-ray pixel detector, the readout circuit comprising: a plurality of analogue front-end circuits ( 631 , 15 ), each of them adapted to read an analogue X-ray exposure value from a pixel ( 25 ) of the pixel detector; a plurality of channels including successive approximation ADCs ( 632 , 621 , 17 ), each analogue front-end circuit ( 631 , 15 ) having an output connected to an ADCs, each ADC being arranged to provided a digital value indicative of the X-ray exposure value of a pixel characterized in that each of the ADCs has a feedback loop comprising a DAC ( 12 , 126 ), the readout circuit comprising a voltage reference unit providing a plurality of reference voltages to the DACs of several ADCs of said plurality of ADCs.