Monolithic active pixel sensors with in-pixel double sampling operation and column-level discrimination

Monolithic Active Pixel Sensors constitute a viable alternative to Hybrid Pixel Sensors and Charge Coupled Devices for the next generation of vertex detectors. Possible application will strongly depend on a successful implementation of on-chip hit recognition and sparsification schemes. These are not a trivial task, first because of very small signal amplitudes (∼mV), originated from charge collection, which are of the same order as natural dispersions in a CMOS process, secondly because of the limitation to use only one type of transistor over the sensitive area. The paper presents a 30 × 128 pixel prototype chip, featuring fast, column parallel signal processing. The pixel concept combines on-pixel amplification with double sampling operation. The pixel output is a differential current signal proportional to the difference between the charges collected in two consecutive time slots. The readout of the pixel is two-phase, matching signal discrimination circuitry implemented at the end of each column. The design of low-noise discriminators includes automatic compensation of offsets for individual pixels. The details of the chip design are presented. Difficulties, encountered from being the first attempt to address on-line hit recognition, are reported. Performances of the pixel and discriminator blocks, determined in separate measurements, are discussed. The essential part of the paper consists of results of first tests performed with soft X-rays from a ⁵⁵Fe source.