Front-end application-specific integrated circuits for the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays High Energy Modular Array and Wide Field Monitor detectors: concept and design

We present the NSX front-end application-specific integrated circuits (ASIC), which has been developed to read charge signals from the High Energy Modular Array and Wide Field Monitor X-ray detectors for the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays mission. The ASIC reads out signals from up to 64 anodes of linear silicon drift detectors (SDDs). When unloaded, the ASIC channel has a charge resolution, expressed in equivalent noise charge (ENC) of similar to 2.8 e(-). Once connected to the SDD anode, we anticipate, for the 80-keV energy range, an ENC of similar to 10.7 e(-) at a leakage current of 2 pA, which corresponds to a full width half maximum of similar to 145 eV at 6 keV once the Fano-limited statistics from charge generation in Si is included. The acquisition is event-triggered, and for events exceeding the threshold, the ASIC measures the peak amplitude and stores it in an analog memory for subsequent readout. The ASIC can also force the measurement of the sub-threshold channels neighboring the triggered channel, including the ones that belong to neighbor chips using bi-directional differential inter-chip communication. Alternatively, the ASIC can measure the amplitudes of all channels at the time of the first detected peak. Additional features include a high-resolution option, a channel power down and skip function, a low-noise pulse generator, a temperature sensor, and the monitoring of the channel analog output and trimmed threshold. The power consumption of the individual channel is similar to 590 mu W, and when including all shared circuits, it averages to similar to 670 mu W/channel.