The AS5911 is an ultra-low noise, 256-channel low power current to digital converter that enables the readout of photodiodes of a CT detector with highest sensitivity. The output currents of 256 photodiodes are converted simultaneously into digital without any dead time resulting in no charge loss on the inputs. The AS5911 combines current-to-voltage integration and A/D conversion for 256-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 AS5911 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 AS5911 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 14 x 14 mm² FBGA package. The on-chip LDOs, reference voltages and a temperature sensor minimize the external component requirements and reduces the bill of material. The AS5911 features on-chip calibration functions for parameters such as offset voltage, linearity, offset charge and leakage current. This allows to automatically set the photodiode and the front-end at their ideal operating point, compensating for offsets and non-linearities of the signal chain.
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. The operating range of AS5911 is specified from 0°C to 85°C.