CMOS PREAMPLIFIER FOR LOW-CAPACITANCE DETECTORS

We present a new CMOS preamplifier and shaper, optimized for charge me asurements with detectors of 0.1-1 pF capacitance. A self-adaptive bia sing scheme with nonlinear pole-zero cancellation allows us to use an MOS device operated in the triode region as the DC feedback element wh ile eliminating nonlinearity and sensitivity to supply, temperature, a nd process variations and accepting up to several mu A leakage current . The circuit is continuously sensitive and requires no external adjus tments to set the feedback resistance. Secondary sources of noise are minimized subject to a power dissipation constraint. Implemented in a 1.2 mu m CMOS process, the preamplifier achieves an ENC of 35 e(-) + 5 8 e(-)/pF at 23 mu s shaping time at a power consumption of about 3.2 mW. The integrated preamp/shaper has 50 ns shaping lime and the ENC is 120 e(-). It has 0.3% nonlinearity over an input dynamic range of 0-5 fC.