GOSSIPO-4: Evaluation of a Novel PLL-Based TDC-Technique for the Readout of GridPix-Detectors

The direct readout of Micro-Pattern Gaseous Detectors (MPGDs) with bare pixel chips introduces the need for a new generation of readout electronics featuring a high spatial granularity as well as a highly accurate time measurement in each pixel. GOSSIPO-4, fabricated in a 130 nm CMOS technology, is a demonstrator ASIC investigating the potential of a new TDC-concept that is based on a chip-wide 40 MHz clock which is complemented by an additional 640 MHz clock. The latter is created upon demand by local oscillators distributed across the pixel matrix. PLL tuning of the local oscillators allows for automatic compensation of frequency fluctuations caused by process parameter, supply voltage and temperature variations. The developed PLL locks within 4 μs and achieves a duty cycle of 50.75% with a time interval error of only 23.4 ps. Mean DNL and INL of the TDC are less than 20% of the time bin size of 1.56 ns under all anticipated conditions.