A CMOS IC for X-ray computed tomography

In x-ray tomography CdTe detectors can be used to convert photons into impulses of charge. A fully integrated charge-sensitive system is implemented in this work, that convert a few femptocoulombs of charge to voltage pulses of around 100-500 mV, while preserving the sharpness of the impulses. On-chip comparator and delay logic convert this analog output into a digital pulse train of fixed, narrow pulse width, ready to be counted by the computer. The op-amps used in the charge-sensitive amplifier have 65Mhz GBW, 80dB DC gain and >70° phase margin. It has a white noise spectral density of 8nV/sqrt(Hz) and a flicker noise corner frequency of 100kHz. The IC is capable of resolving a minimum ion impulse rate of 5-10 Mhz and a minimum input charge of 1 femptocoulomb.