Waveform digitization with programmable windowed real-time trigger capability

Abstract

A transient waveform digitizer circuit with continuous sampling capability and real-time programmable windowed trigger generation has been fabricated and tested. Designed in 0.25 μm CMOS, the digitizer contains a circular array of 128 sample and hold circuits for continuous sample storage, and attains 2 GHz sample speeds with up to 2 GHz analog bandwidth. Sample clock generation adopts a semi-synchronous approach, combining a phase-locked loop for highspeed clock generation and a high-speed fully-differential shift register for distributing clocks to all 128 sample circuits. Using two comparators per sample, the sample voltage levels are compared against reference levels that are set via per-comparator digital to analog converters (DACs). The 256 per-comparator 5-bit DACs compensate for offsets and allow for fine reference level adjustment. The comparator results are matched in 8-sample wide windows against up to 72 programmable patterns in realtime using a programmable logic array. Each trigger window is 8 samples-wide, overlapped sample by sample in a circular fashion through the entire 128 sample array. A trigger is flagged within 7 ns if there is a match, after which on-chip digitization can proceed via 128 parallel 10-bit converters.