Fully Integrated Constant Fraction Discriminators (CFDs): Two Implementations in Submicrometer CMOS Technologies

The need for the precise measurement of the time associated with the passage through the matter of a quantum of radiation is of primary importance in a number of scientific fields and motivates the development of very specific and highly performing detector systems. The integration of constant-fraction-discriminators (CFDs) suitable for multichannel mixed-mode ICs represents an attractive approach to obtain precise timing. Issues related to area occupation, power consumption, and timing accuracy are discussed in detail. Two CFD architectures were developed in submicrometer CMOS technology and integrated in pixelated application-specific integrated circuits (ASICs). The first implementation was optimized for a 200- ${\mu }\text{m}$ -thick silicon sensor with a 3-ns collection time and an input capacitance of 200 fF. The charge produced ranges from 1 to 10 fC. The fabricated prototype was tested by injecting test pulses and was able to reduce the time walk due to amplitude variations by a factor 4, resulting in an overall time resolution better than 100 ps rms. The second presented architecture is suitable for detectors with gain, providing signals in the range of charges between 80 and 400 fC and with input capacitance up to 1 pF. A large-scale ASIC with 1024 channels was produced and laser tested. It was able to correct signal variations, providing a time resolution better than 50 ps rms.