A low-power low-noise synchronous pixel front-end chain in 65 nm CMOS technology with local fast ToT encoding and autozeroing for extreme rate and radiation at HL-LHC

A low-power and low-noise synchronous front-end chain in a commercial 65 nm CMOS technology suitable for the future pixel upgrades at the CERN Large Hadron Collider (LHC) is presented. A shaper-less Charge-Sensitive Amplifier (CSA) with constant current feedback provides triangular pulse shaping for linear Time-over-Threshold (ToT) charge measurement. The sensor leakage current is compensated by the same feedback network. A track-and-latch voltage comparator is adopted for the hit discrimination. The hit generation is synchronized with a 40 MHz clock, minimizing time-walk issues in the time-stamp assignment. Fast ToT charge encoding up to 8-bit resolution can be retrieved at the pixel level exploiting a high-frequency self-generated clock signal. This is obtained by turning the latch into a voltage-controlled oscillator (VCO) using asynchronous logic. Pixel-to-pixel threshold variations are compensated by means of an autozeroed scheme, thus avoiding the need of a on-pixel D/A converter. An array of 8 × 8 cells with 50 μm × 50 μm pixel size has been prototyped. Design specifications, implementation and test results are discussed.