A monolithic, 2 μm CMOS constant-fraction discriminator for moderate time resolution systems

A monolithic, 2 μm CMOS constant-fraction discriminator (CFD) was developed for use with moderate time resolution (about 4 ns FWHM) scintillation detectors such as BGO. The circuit was fabricated through the metal-oxide-semiconductor implementation service (MOSIS) using a double-metal, double-polysilicon digital process. The measured CFD walk is 1.4 ns total for input signals of -200 to -2000 mV with risetimes of 20 ns. The measured CFD noise jitter is 70 ps FWHM for -200 mV, 20 ns risetime signals. Measured timing performance of the CFD circuit with a BGO scintillation detector shows good performance for detectors with timing resolutions in the 4 ns FWHM range. A general-purpose comparator, included on the same die as the CFD for separate evaluation, has measured response times of 32 ns for a 50 mV overdrive input and 23 ns for a 500 mV overdrive input. Both the CFD and the general-purpose comparator have application in large multichannel systems, including positron-emission tomography (PET), where circuit size and power consumption are limited for each system channel