Speaker
Description
Spectral X-ray imaging based on single-photon-counting (SPC) detectors enables energy-resolved detection of individual X-ray photons, thereby offering substantial reductions in radiation dose together with improved tissue contrast, higher spatial resolution, and enhanced material decomposition capabilities in imaging modalities such as mammography and computed tomography (CT) [1]. However, achieving high spatial resolution remains a critical challenge in readout integrated circuit (ROIC) design [2]. To address this issue, we present, for the first time, the design and preliminary experimental results from a CMOS ROIC with frequency-division-multiplexed (FDM) readout [3] tailored for direct-conversion X-ray sensors such as amorphous selenium (a-Se) [4] and cadmium telluride (CdTe).
Use of FDM readout enables our ROIC, fabricated in a 1P6M 1.8-V-supply 180-nm CMOS process, to achieve a compact 50×50-mm2 pixel pitch without compromising count rate. Our novel approach relocates the analog-to-digital converter (ADC) in the SPC readout chain, which is normally located within the pixel, to a location outside the pixel array. This organization allows a single high-speed ADC to serve an entire column containing eight pixels, where the output from each pixel in the column is frequency-translated to one of seven carrier frequencies f1-f8, and then summed in the current domain and digitized by the column ADC.
As shown in Fig. 1, each pixel in a column consists of a charge-sensitive amplifier (CSA) followed by a CR-RC shaper which feeds a passive mixer. The mixer up-converts the shaped voltage pulse, which is then converted to a current Iout by a transconductance-amplifier (Gm) stage driving the column line. Output currents from each pixel Iout1-Iout8 are then summed before being converted back to a voltage by a transimpedance amplifier (TIA) and processed by an 8-bit 200-MS/s pipelined ADC.
As shown Fig. 2, experimental characterization of our fabricated ROIC prototype validates our proposed FDM-readout concept, demonstrating that independent photons from an Am241 source can be accurately isolated and reconstructed from the multiplexed data stream via digital down-conversion. In addition, our system achieves robust noise performance with an average equivalent noise charge (ENC) of 96 e- for a single pixel and 306 e- for the entire column (which includes eight pixels). Our shared column-parallel pipelined ADC achieves an effective number of bits (ENOB) of 7.3 bits and a peak signal-to-noise ratio (SNR) of 45.8 dB. Our ADC also exhibits excellent linearity with differential non-linearity (DNL) less than 0.5 LSB and integral non-linearity (INL) maintained within +/-1 LSB. Furthermore, the column-level TIA maintains a 200-MHz closed-loop bandwidth. Critically, our ROIC meets the high flux target count rate of 108 photons/(mm2×s), while adhering to a strict power budget of approximately 60 µW per pixel, which is essential to prevent a-Se sensor crystallization due to excessive heating [4]. These quantitative results establish FDM as a viable and scalable architecture for next-generation line-scanning SPC X-ray imagers.