Ultra low noise CMOS preamplifier-shaper for X-ray spectroscopy

We report here an Ultra Low Noise CMOS charge-sensitive amplifier (CSA) and a CSA-Shaper for X-ray spectroscopy, suitable for a small anode capacitance, low leakage current solid state detector. The continuously sensitive CSA and CSA-Shaper use a MOS transistor MF to discharge the integration capacitance; a novel self-bias circuit biases MF in the G Ohm region (subthreshold region) while tracking process, temperature and supply voltage variations. A T-Network in the DC-feedback improves the CSA gain linearity (within 0.1% up to 1.8 fC input charge). The CSA-Shaper features a novel Adaptive Pole-Zero cancellation scheme: the Zero in the Shaper adapts itself statically and dynamically to cancel the CSA Pole. The noise performance of the CSA has been measured: with no detector connected and MF pinched, the room-temperature equivalent noise charge (ENC) is 9e rms at 12 mu s shaping time. When connected to a cooled detector (T = - 75 degrees C), with MF in pinch off, an ENC of 13e rms is achieved (FHWM = 111 eV) at 2.4 mu s shaping time. This is the best reported energy resolution ever obtained with a CMOS preamplifier. (C) 1998 Elsevier Science B.V. All rights reserved.