CMOS preamplifier for low-capacitance detectors

We present a new CMOS preamplifier and shaper, optimized for charge measurements with detectors of 0.1-1 pF capacitance. A self-adaptive biasing scheme with nonlinear pole-zero cancellation allows us to use an MOS device operated in the triode region as the DC feedback element while eliminating nonlinearity and sensitivity to supply, temperature, and process variations and accepting up to several mu A leakage current. The circuit is continuously sensitive and requires no external adjustments 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(-) + 58 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.