ams: 26-bit, 128-channel current-input ADC for photodiode arrays

ams released the AS5900, a current-to-digital converter for use in computed tomography scanners which offers ultra-low noise, very high resolution and excellent linearity. The outstanding analog performance of the AS5900 will enable new CT scanners to produce sharper, more detailed images. Medical CT scanners equipped with the AS5900 will feature an improved X-ray detector module, and will help physicians to produce more accurate diagnoses while subjecting patients to lower doses of radiation.

The AS5900 detector interface chip for CT X-ray scanners sets new performance benchmarks in crucial parameters:

  • Ultra-low noise: As little as 3,200 electrons for a full-scale range at an input current of 1 µA and 30 pF input capacitance. The circuit’s very high signal-to-noise ratio allows the flexible use of any pixel size, enabling it to digitize extremely low-current outputs from the latest high-sensitivity photodiodes. As a result, the CT scanner can render more detailed images. Furthermore, low noise allows CT equipment to maintain high spatial resolution while reducing the X-ray dose to which the patient is exposed.
  • Very low power dissipation: Power dissipation as low as 1mW per channel reduces the requirement for cooling capability in the CT scanner, lowering the overall system cost.
  • High resolution: The 26-bit sigma delta ADC integrated in the AS5900 produces a richer grayscale palette for sharper and more detailed images.
  • High linearity: ±250 ppm of Read inputs (±1 ppm of full-scale range) to give improved contrast, accurately rendering slight variations in image brightness.

The new ams detector chip provides detailed motion-free medical images while operating at very high acquisition rates: the adjustable integration time can be set as low as 50 µs.

The configurable design of the AS5900 gives it the flexibility required for integration into different types of end equipment that use a multiple-slice photodiode array. The chip is ideal for use not only in medical equipment, but also in industrial and security CT scanners. Equipment designers can configure or adjust gain setting, power modes and clock frequency.

The AS5900 integrates all the functions of a current-input ADC to minimize the requirement for external components. It includes an internal reference voltage and temperature sensor. This reduces the bill of materials, thus lowering system cost. The AS5900 also provides a standard LVDS interface to a host FPGA, running at a data rate of up to 80 Mb/s for optimized power consumption. The AS5900 is available in a 10 x 10mm² BGA package for sampling now.


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