A 10-Mcps, 0.5-μm CMOS constant-fraction discriminator having built-in pulse tail cancellation

A 0.5-μm, fully-integrated CMOS, constant-fraction discriminator (CFD) is presented for 10-Mcps operation. The CFD utilizes a timing-shaping filter to provide amplitude insensitive time discrimination without delay lines or other external components. Continuous-time baseline restorers are used to cancel CMOS dc offsets and provide signal tail cancellation for both the arming and constant-fraction signals. Near total signal tail cancellation at 100 ns permits operation at count rates up to 10 Mcps. Measured intrinsic timing resolution is 40 to 90 ps FWHM for a 3.7:1 input dynamic range, and measured walk is 20 ps for a 2.7:1 dynamic range increasing to 80 ps for a 3.7:1 dynamic range. Measured timing resolution for a LSO/PMT detector against a plastic/PMT detector is 1.00-ns FWHM a low count rates increasing to 1.27-ns FWHM at 6.0 Mcps. The CFD is part of a 0.5-μm CMOS front-end integrated circuit designed for large area, high count rate LSO or BGO detectors used in PET medical imaging.