CASCA: A readout ASIC for a TPC based X-ray polarimeter

Abstract

The paper presents a prototype ASIC (CASCA: Charge Amplifier with Switched Capacitor Array) for a TPC based X-ray Polarimeter (XTP). It measures two dimensional photoelectron tracks generated by the incident X-ray photons with one dimensional strip readout. The other dimension is calculated by the drift time from the signal waveform. In order to achieve 100 μm track resolution fine pitch strips are used, requiring high density readout electronics with 20 MSPS waveform sampling capability. CASCA integrates 32 channel full function signal processing circuits and is fabricated in a 0.18μm CMOS process. Each channel consists of a charge sensitive preamplifier, a CR-RC shaper with a peaking time of 25 ns, a baseline holder, a discriminator and a 64-cell switched capacitor array (SCA). The analog front-end and the SCA circuits have been implemented and verified in two separate chips in the previous prototypes. Several modifications have also been made, such as using native NMOS capacitor to store the samples and changing the structure of the switched capacitor circuits. The test results of CASCA show that the equivalent noise charge (ENC) is about 1500e with 14.5pF input capacitance. The INL (Integrated Nonlinearity) is better than 0.6% and the center-of-gravity time jitter has been measured to be no more than 3 ns.