Design of the analog front end circuit for X-ray detectors
Roy, Subhash Chandra
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The Thesis presents a novel idea to efficiently read out the value corresponding to incident X-Ray, from X-Ray sensor. A system level solution has been proposed which is unique in itself in terms of approach. A simple design of analog front end circuit for 64 channels, consisting of Charge Sensitive Preamplifier (CSP), Pulse Shaping Amplifier (PSA), Peak Detector, subtractor, Mux and ADC has been proposed. In CSP, Transmission Gate (TG) has been used, in parallel with integrating capacitor, where the NMOS is operating in weak inversion, when TG is supposed to be off. It fulfils the requirements like posing very high ac resistance, providing alternative path for DC leakage current signal, discharging integrating capacitor quickly etc. An amplifier cum level shifter has been used to match the output DC level of CSP with input DC level of PSA. PSA has been implemented as a 4th order Bessel-Butterworth low pass filter, which provides good step response, and hence output is obtained with negligible peaking. High pass filter hasn’t been used to avoid low frequency signal loss. A subtractor has been proposed after the peak detector, which is taking care of offset voltages and low frequency noise. This system till the output of shaper is providing a resolution of 1.7% against the specification of 3%.
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