A readout ASIC for SPECT

A new low-noise multichannel application specific integrated circuit (ASIC) has been designed for use in nuclear medical imaging single photon emission computed tomography (SPECT), and prototypes have been produced and tested. The ASIC has 128 charge sensitive amplifiers (CSAs) followed by shapers, threshold discriminators, and sample-and-hold. The ASIC delivers analog signal amplitude and digital address from channels with signals above threshold, and optionally allows one to read signals from any channel using a random-access readout. The total power consumption is 358 mW. The ASIC parameters have been specified and measured as follows: each amplifier can measure charge within a linear dynamic range from -40000 e to +40000 e. The preamplifier noise is 56 e+11.6 e/pF at 10 μs shaping time. The ASIC is ideally suited for readout of radiation sensors with a few pF capacitance and less than 100 pA dc input current. The ASIC has been tested with silicon diode pad sensors: the diode had a capacitance of ≈1 pF and a leakage current of 20 pA. We measure an equivalent noise charge of 85 e, and achieve a triggering threshold of 800 e. Energy spectra from radioactive isotopes have been measured with energy resolution of 823 eV FWHM for 5.9 keV X-rays.