ams AS5900 CT Detector Interface

The AS5950 is a sensor chip for 16-slice CT detectors that combines the photodiodes and the readout circuit on a single CMOS chip. This sensor solution, which includes an array of photodiodes with ultra-low dark current and a 64-channel ADC side-by-side, allows the assembly of the pixel array on three adjacent edges of the device. Two AS5950 ICs can be placed in Z-direction enabling the design of 16-slice detectors for cost optimized CT machines. The total sensor dimension in Z-direction is selectable between 16 or 32 mm, enabling two modes of operation. The adaptive array concept allows either a high resolution or a large sensor size on the detector. Pixel dimensions can be customized on request. The available standard product comes with a pixel dimension of 0.98x0.98 mm². The sensor can be directly assembled on a substrate using a wire bonding process for manufacturing of a CT module. Superior image quality can be achieved because the input-related noise is very low. In high-resolution mode a typical noise of 0.20 fC can be reached including the photodiode. This ultra-low noise figure is the result of the small capacitance of the photodiode, and the very low parasitic capacitance of the interconnect between the photodiode and its corresponding ADC channel. The low power consumption of typ. 0.65 mW per channel reduces self-heating effects and reduces the overall cost of cooling the system. The digital data readout can be accessed via SPI interface. It is also used to configure parameters such as pixel resolution, input current range, active sensor area and enabling the calibration mode. An integrated temperature sensor enables monitoring of the junction temperature. Featuring on-chip photodiodes, the AS5950 offers a cost-optimized solution and is delivered as die on foil.

The AS5912 is an ultra-low noise, 512-channel low power current to digital converter that enables the readout of photodiodes of a CT detector with highest sensitivity. The output currents of 512 photodiodes are converted simultaneously into digital without any dead time resulting in no charge loss on the inputs. The AS5912 combines current-to-voltage integration and A/D conversion for 512-channels in one device. The ADC architecture with its low-noise integrators, coarse quantizer, sample-and-hold and fine quantizer guarantee lowest input related noise of maximum 0.29 fC (low noise mode) at an input current range of 0.5 µA. The input current range of the ADC is selectable between 0.5 µA and 1 µA.The AS5912 supports two supply concepts: a single analog and true ground supply, in both modes the photodiodes are zero biased for ultra-low dark current of the photodiode. In true ground mode, the AS5912 operates at ±1.5 V dual analog supply while in single supply mode only a single analog supply of +3.0 V is needed. The device detects automatically the chosen supply concept. No external components are required for supply decoupling. To reduce PCB area and simplify the assembly process, decoupling capacitors for supply and reference voltages are integrated in the 23 x 15 mm² FBGA package. The on-chip LDOs, reference voltages and a temperature sensor minimize the external component requirements and reduces the bill of material.Full-scale range, integration time and power mode are configurable over a SPI-compatible serial interface as well as the digital output modes of the LVDS interface. In low power mode, the power dissipation can be minimized down to 1.25 mW per channel for reduced self-heating of the CT detector. The adjustable integration time down to 51 µs allow very high frame rates.