A Constant Fraction Discriminator with Shape-Agnostic Fraction Triggering and Sub-ns Walk for the Solar Probe Analyzer for Ions

Electrostatic analyzers like the Solar Probe Analyzer for Ions can use time-of-flight techniques to determine the ion composition of space plasmas. The conversion of trigger events to their digital equivalent is a central component of any timing-based front end, with integrated solutions increasingly desirable for constrained size, weight, and power budgets. Conventional analog methods of pulse discrimination introduce timing walk or are limited to a narrow set of pulse shapes, while digital methods require impractically high sample rates for the allowable power consumption. This work presents an integrated constant fraction discriminator with theoretically zero timing walk and a well-defined and programmable trigger fraction that does not depend on input pulse shape. The test chip was fabricated in 0.18 mu m bulk CMOS with 3 mA of current drawn from an internally regulated 1.8 V supply, and a measured timing walk of 601 ps over a 10x change in input pulse amplitude with a worst case jitter of 743 ps.