ams and Osram provide high-resolution images for computed tomography in the medical and security sectors

     ams Osram, the world's leading optical solutions provider, has expanded its sensor chip portfolio with the launch of a 32-layer solution for computed tomography (CT) scanners. CT equipment is widely used in many fields such as medical, industrial or security. In all of these applications, the detector is the deciding factor in the development of a cost-effective CT scanner. Jose Vinau, head of the Medical and CMOS division at ams Osram, said: “The AS5951 sensor chip from ams Osram simplifies detector assembly. It integrates a high-sensitivity photodiode array and readout circuitry in a single CMOS chip. In comparison, the AS5951 provides higher resolution images for the same sensing area."

     Compared with the previous generation AS5950, the new generation mainly improves the spatial resolution of the same detector coverage. Sub-millimeter isotropic pixel pitch increases image resolution. The AS5951's photodiode array consists of 16 x 8 pixels with a pitch of 0.98 x 0.98 mm², an improvement over the 0.98 x 1.96 mm² pitch of its predecessor. Therefore, two AS5951 sensors can be used to achieve a 32-layer detector with a total size of 31.23 mm and double the spatial resolution. To meet higher pixel density requirements, the number of channels in the A/D converter (ADC) has been increased from 64 to 128. Pixel size can be customized in a short development cycle to meet customer needs.

     Another factor that affects image performance is the low dark current of 1 pA maximum, which is achieved with an auto-calibrated zero bias voltage near the photodiode. This parameter is 90% lower than AS5950 and enables better low dose performance. Input-dependent noise performance is a key factor affecting image quality. Because of the short connection between the photodiode and ADC channel, the AS5951 has very low noise performance, which can improve low-dose performance.

     The AS5951 has two modes of operation: high-resolution mode and low-dose mode. In high-resolution mode, the maximum noise is 0.30 fC when the input current range is 200 nA. In low dose mode, at a noise level of 0.43 fC, two pixels are merged into one larger pixel, resulting in improved signal-to-noise performance and lower dose levels. High linearity of ±600 ppm (including photodiode) also ensures artifact-free images. The shortest integration time (200 µs) allows fast rotation of the CT detector. Digital data can be accessed and read through the SPI interface.