Test of a fully integrated CMOS preamplifier for HPGe detectors

A CMOS fully integrated charge preamplifier for nuclear physics spectroscopy measurements, designed to work with germanium detectors in the Gerda experiment is tested. The electronics is composed of two distinct circuits: a low noise charge sensitive preamplifier (CSA) and a fully differential low impedance coaxial cable driver. A couple of passive elements (few resistors and the feedback capacitor) are not yet integrated, in order to make the debug process easier and to fit their values to the specific needs of different set-up configurations. The preamplifier operates at +2.5 V / -2.5 V power supplies, with 50 mW power consumption and 8 V differential output bipolar dynamic range at the driver side of the 50 ohm impedance cable. Up to now, we tested it in an experimental set-up consisting of an encapsulated p-type germanium detector operated at liquid nitrogen temperature by means of a custom Dewar container. Reliability and robustness of the CMOS electronics and mounting set-up have been verified, e.g. in terms of immunity to normal operation of the high voltage detector power supply and integrity of IC bonding wires in contact with liquid nitrogen. Long term stability of the preamplifier gain, although operating at almost constant temperature, has also been verified by overnight measurements. Overall noise performance is worse than expected from previous set-ups with simulated detector capacitance, due to relatively larger detector capacitance and leakage current contributions previously not completely taken into account.