Improved readout IC for multi-energy X-ray imaging with linear CZT pixel arrays

We have developed a new, improved version of the XENA (X-ray ENergy-binning Applications) readout IC for solid-state x-ray detector arrays, which we call XENA-2. This IC consists of 32 readout channels, each with charge-sensitive input amplifier, two-stage gain amplifier, and five comparators followed by 16-bit pulse counters. The resistive feedback of the input amplifier is provided by a transconductance circuit with continuously variable, user-controlled transconductance, which allows the amplifier’s shaping time to be varied between 100 to 150 ns and 4 μs. Two input amplitude ranges, 7 fC and 30 fC nominal (200 keV and 800 keV, respectively, for Cadmium Zinc Telluride), can be selected by switching different capacitors into the feedback path. The gain amplifier design includes a five-bit gain adjustment and a ten-bit R-2R offset DAC. The five comparator threshold voltages are supplied externally and are common to all channels; digitally controlled threshold adjustments at each comparator allow to compensate for process variations. Readout of the counters, over a 16-bit data bus, takes approximately 20 μs. Two additional analog-only channels (that is, without the comparators and counters) are provided, one at each end of the channel array; they are used for test purposes and to improve the uniformity of the full channels. Compared to XENA, this new IC’s main improvement is significantly reduced noise, which allows for lower comparator thresholds and increased count rates. XENA-2 is fully pin-compatible with XENA and replaces it as the readout IC used in NOVA’s NEXIS detector system.