The development of a readout ASIC for TPC based X-ray polarimeter

A dedicated readout ASIC has been developed for a TPC based X-ray polarimeter. It consists of a charge sensitive preamplifier, a CR-RC shaper and a 64-cell switched capacitor array (SCA) for each channel. The signal charge distribution along the time axis can be extracted from the recorded waveforms. The first prototype has been implemented in two separate chips: the front-end part and the SCA part. The front-end part consists of a charge sensitive preamplifier and a CR-RC shaper with a peaking time of 25 ns. Simple digital signal processing algorithm such as de-convolution can be easily applied. The SCA structure is adopted for its low power consumption. The sampling frequency and the resolution is designed to be above 20MSPS (Mega-samples per Second) and 10bit, respectively. High density MOS capacitors are used for storing the samples. Careful biasing and readout scheme are used to reduce the non-linearity caused by the voltage-dependent capacitance. The SCA has differential current outputs and can be readout at 40MHz. Each ASIC has integrated 32 channels and is fabricated in a 0.18μm CMOS process. The preliminary test results show that both chips are fully functional. The noise of the front-end has been measured to be 455.2e+70.43e/pF. The write control logic of the SCA can work at 50MHz. The typical noise of a cell in the SCA is 1.25mV, meaning the static readout resolution of 9.6 bit. A maximum INL of the connected two chips is 0.27%.