The TDCpix ASIC: High rate readout of hybrid pixels with Timing Resolution Better than 200 ps

Abstract

The TDCpix is a novel readout ASIC for hybrid pixel detectors with emphasis on timing. It is designed to meet the requirements of the Gigatracker, the kaon spectrometer of the NA62 experiment at the CERN Super Proton Synchrotron. The Gigatracker consists of three tracking and time-tagging hybrid pixel modules. A time resolution better than 200 ps is required. The silicon pixel sensors will be 60×27 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 200 µm thick. A pixel size of 300×300 µm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> will provide the required position and momentum resolution. Each sensor will be read out by 2×5 TDCpix ASICs, connected by flip-chip bonding. The main capabilities of the TDCpix chip will be to detect and time stamp more than 130 million charged particle hits per second (>0.8 MHz/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) with a timing resolution below 200 ps (RMS). This contribution describes the completed and final design of the TDCpix ASIC. The chip has been designed in a 130 nm CMOS process. It combines a matrix of 45×40 pixel channels with amplifying and discriminating circuits and a complex peripheral region including an array of TDCs based on DLLs, four high speed (3.2 Gb/s) data serializers, a low-jitter PLL, bandgap references, DACs, readout and control circuits. Mandatory correction of the time-walk of the discriminator leading edge is based on the measurement of Time over Threshold. The laboratory characterization of a prototype chip demonstrated a resolution better than 75 ps RMS for injected charges larger than 2 fC and constant pulse shapes. The achievement of the target timing resolution with real particles has been demonstrated in beam tests using prototype chips and prototype hybrid modules.