Study of the effects of parasitic capacitance on large integrated feedback resistors for charge-sensitive preamplifiers

A study of the impedance and noise of an integrated poly-silicon resistor with distributed capacitive coupling to bulk is presented. The system is analyzed by using a closed-form calculus which produces non approximated results. In particular, deviations from ideal conditions are shown when that kind of resistor is used as a continuous-discharge feedback device for charge-sensitive preamplifiers. The effect of using a feedback resistor with capacitive coupling to bulk is an evident deformation of the shape of the signals. An innovative technique is presented, which allows overcoming the problem of the distortions in the CSP output signals. Several n-wells are placed under the resistor and driven with an auxiliary low-impedance divider which minimizes the voltage variations on the plates of the parasitic capacitors. In this way the pulses provided by the charge-sensitive preamplifiers show again a clean R-C exponential-decay shape and are so optimized for signal shaping and adequate for high-resolution spectroscopy.