A low-power low-noise CMOS charge-sensitive amplifier for capacitive detectors

In this paper, the design of a new low-power low-noise charge-sensitive amplifier (CSA) is presented. The proposed CSA is intended for capacitive sensor readout circuits such as interface circuits for solid-state detectors used in medical imaging and X-ray spectroscopy. A comprehensive noise analysis of readout systems that consist of a CSA followed by an RC-CR pulse shaper is presented. To facilitate predicting the noise behaviour of the system, the equivalent noise charge (ENC) equations are derived analytically. The readout circuit is designed and laid out in a 0.13-μm CMOS process. Post-layout simulations show that the conversion gain of the CSA with a 20 fF feedback capacitor is 37.5 mV/fC. The estimated ENC of the readout system is 38 e̅-rms at a 1 μs peaking time with a detector capacitance of 0.5 pF and a leakage current of 50 pA. The integral nonlinearity of the CSA is less than 0.74% for 0.5-3 ke̅. The open-loop gain of the amplifier is ~ 80 dB and the gain-bandwidth product is about 345 MHz. The CSA occupies 0.0021 mm² and consumes 37.5 μW from a 1.2 V supply.