CMOS PREAMPLIFIER WITH HIGH LINEARITY AND ULTRA-LOW NOISE FOR X-RAY SPECTROSCOPY

We report here an ultra low noise charge sensitive monolithic CMOS amp lifier (CSA) suitable for small anode capacitance (200fF), low leakage current solid state detectors. The CSA is continuously sensitive, the charge from the input node being drained by a feedback transistor Mf biased as a resistor with effective values in the G Omega range. This very high value was achieved by a novel scheme which tracks threshold variations, and power supply and temperature fluctuations. A good line arity of the CSA conversion gain is achieved (<0.1% up to 1.8fC input charge) by inserting a voltage divider between the output of the CSA a nd the source of Mf. The equivalent noise charge (ENC) of the CSA is e qual to the theoretical lower limit imposed by the flicker noise. The circuit has been fabricated in two different CMOS technologies. With n o detector connected, we measure a room-temperature ENC of 9 e(-) rms at 12 mu sec shaping time. When coupled to a cooled detector a FWHM of 111 eV is obtained at 2.4 mu sec shaping, corresponding to an ENC of 13e(-) rms. This is the best reported energy resolution ever obtained with a CMOS preamplifier.