Qpix v.1: A high speed 400-pixels readout LSI with 10-bit 10 MSps pixel ADCs

Qpix is a multi-functional CMOS pixel readout LSI, which can directly measure the total deposited charge, as well as time of flight (TOF) and time over threshold (TOT) in each pixel. Qpix v.1 is the newest version of Qpix. The chip is designed and fabricated in a six-metal 0.18μm commercial CMOS process. It has a dimension of 5 mm×5 mm and possesses 400 identical pixels each with an area of 200μm×200μm. Except the 130μm×140μm active circuitry, the pixel pad occupies almost all the other area. This large pixel pad in each pixel can be used as an active anode to directly collect the charges generated in the gas chamber such as a micro-pattern gas detector based time projection chamber. The successive approximation register ADC in each pixel is optimized to 10 bits and 10 MSps without any increase of area. Also, offset calibration technology is applied to both the amplifier and the comparators to reduce the mismatch between pixels. The TOT counter is designed to have 8 bits with an overflow control function and the TOF counter is designed to have 14 bits. The preliminary electrical measurement results show that Qpix v.1 can measure TOT and TOF with an accuracy of 10 ns. ADC codes show that the monotonicity is achieved. Compact readout structure in Qpix v.1 provides both parallel and serial readout modes, which guarantees that all the data in one chip can be read out at a high data rate of 240 Mbps. Thus the total time to read out the data in the 400-pixel matrix is 2.6μs in parallel readout mode and 54μs in serial readout mode. The power consumption has been reduced to 187.5μW per pixel. More detailed and accurate measurements of Qpix coupling with gas electron multipliers are in progress.