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 (200fF), low leakage current solid state detectors. A self adaptive bias circuit for the MOS feedback device establishes the static feedback resistance in the G Omega range while tracking the threshold variations and power supply and temperature fluctuations. The linerity 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 1.2um 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 Omega output driver are included on the die.