CMOS preamplifier with high linearity and ultra low noise for X-ray spectroscopy

We report here an ultra low noise charge sensitive monolithic CMOS amplifier (CSA) suitable for small anode capacitance (200 fF), 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 GΩ range. This very high value was achieved by a novel scheme which tracks threshold variations, and power supply and temperature fluctuations. A good linearity of the CSA conversion gain is achieved (<0.1% up to 1.8 fC input charge) by inserting a voltage divider between the output of the CSA and the source of Mf. The equivalent noise charge (ENC) of the CSA is equal to the theoretical lower limit imposed by the flicker noise. The circuit has been fabricated in two different CMOS technologies. With no detector connected, we measure a room-temperature ENC of 9 e^- rms at 12 μsec shaping time. When coupled to a cooled detector a FWHM of 111 eV is obtained at 2.4 μsec shaping, corresponding to an ENC of 13 e^- rms. This is the best reported energy resolution ever obtained with a CMOS preamplifier