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

We present an ultra low noise charge preamplifier suitable for small capacitance (200 fF), low leakage current solid state detectors. A self adaptive bias circuit for the MOS feedback device establishes the static feedback resistance in the GΩ range while tracking the threshold variations and power supply and temperature fluctuations. The linearity of the gain versus input charge has been improved by means of a voltage divider between the output of the charge-sensitive amplifier and the source of the feedback transistor. With the preamplifier alone, we measure a room-temperature equivalent noise charge (ENC) of 9 e^- rms at 12 usec shaping time. When coupled to a cooled detector a FWHM of 130 eV is obtained at 2.4 usec shaping, corresponding to an ENC of 16 e^- rms. This is the best reported resolution obtained with a CMOS preamplifier. The circuit has good linearity (<0.2%) up to 1.8 fC. Since the preamplifier´s ENC is limited by flicker noise, we fabricated the circuit in two 1.2 μm CMOS technologies. Device measurements allow us to compare the 1/f noise behavior of each foundry. In addition to the preamplifiers, a 1 us shaper and a 50 Ω output driver are included on the die