Design of pixel electronics based on asynchronous self-reset approach with floating-point output representation for high dynamic range imagers

A readout circuit suitable for multi-channel preamplifiers for pixellated detectors, hybrid detectors or thin film on asic imagers, for applications requiring large dynamic range such as computed tomography is discussed. The circuit implements an asynchronous self-reset with residue conversion scheme combined with a floating point representation of the input current. This solution allows to reach a very high dynamic range with good linearity while ensuring a compact output format. In particular, in the present implementation the input current range extends from 50 fA up to 820 nA corresponding to a 144 dB dynamic range which is equivalent to a 24-bit code. However the proposed scheme only uses 16 output bits for the floating point representation with 12-bit constant relative resolution: 12 bits serve for the output value itself and 4 for storing the position of the transition point between integer and fractional part. In addition, an analytical study of the achievable imaging performances shows that no significant degradation of the SNR is expected for the 12-bit constant relative resolution implementation with respect to the full 24-bit resolution scheme. Finally, the main building blocks of the circuit are analyzed in detail and their characteristics are put in relationship with the overall system performances.