A CMOS time-of-flight system-on-a-chip for spacecraft instruments

A CMOS time-of-flight (TOF) system-on-a-chip (SoC) for precise time-interval measurement with low power and high rate has been developed. Microchannel plate electron multipliers or photomultiplier tubes (PMTs) typically produce the start-stop delta-Ts of radiation events to be processed. The TOF chip includes two constant fraction discriminators (CFDs) and a time-to-digital converter (TDC). The CFDs interface to start and stop anodes through two simple preamplifiers and perform the signal processing for time walk compensation. The TDC digitizes the time difference with reference to an off-chip precise oscillator. The design is full custom in both the analog and the digital sections. A first version of the TOF chip developed in a 0.8-μm CMOS process achieved CFD time walk of ∼350 ps for an input amplitude dynamic range of 40 dB and a combined CFD+TDC time jitter of ∼180 ps. The total power dissipation was ∼20 mW at an event rate of 100 K/s and ∼30 mW at a rate of 1 Meg/s. This chip is part of the high-energy neutral atom instrument onboard the NASA/IMAGE spacecraft launched in March 2000. It is selected for many other instruments, including the particle sensor, the fast plasma sensor, and the laser altimeter of Messenger for the Mercury exploration to be launched in 2004.