Optimization and Performance Characterization of the Second Generation Fermilab Constant Fraction Discriminator Readout ASIC

TL;DR

This work presents the optimization and comprehensive performance characterization of the second-generation FCFD1.1 ASIC for AC-coupled LGAD strip sensors. By employing a constant fraction discriminator with a redesigned two-stage front-end and an on-chip amplitude readout, the authors achieve amplitude-insensitive timing with a target of $40~\mathrm{ps}$ and spatial interpolation yielding about $15~\mu\mathrm{m}$ resolution across 1 cm strips with $500~\mu\mathrm{m}$ pitch. A dedicated RC-parameter extraction methodology for AC-LGAD sensors informs accurate sensor modeling, enabling precise CFD operation and robust timing across channels. Beam tests and lab measurements demonstrate these capabilities, highlighting the ASIC’s suitability for high-precision timing and tracking in future collider detectors and the ePIC TOF system at the EIC. The results suggest a clear path toward a six-channel mixed-sensor prototype with integrated ADC/DAC/I2C for system-level testing.

Abstract

We present the optimization and performance characterization of the second-generation Fermilab Constant Fraction Discriminator ASIC (FCFD), designed for the readout of AC-coupled low-gain avalanche detector (LGAD) strip-sensors. The FCFD is explicitly engineered to be insensitive to signal-amplitude variations, thereby removing the need for time-walk correction that is required in other LGAD time-stamping readout ASICs. This updated version, referred to as FCFD1.1, incorporates several enhancements over the first iteration to address key challenges in AC-coupled LGAD front-end design. We outline the primary readout-ASIC design considerations for these applications, describe the methodology used to evaluate critical sensor and system parameters, and summarize the additionally implemented features. Performance measurements using injected charge signals and minimum-ionizing particles in test-beams demonstrate a time resolution of approximately 40 ps and a position resolution of roughly \SI{15}{\micro\meter} when tested with beam particles uniformly over the active area of the sensor.

Figures (15)

• Figure 1: A cross-sectional view of a typical AC-LGAD device.
• Figure 2: Photograph of an example Hamamatsu AC-LGAD strip sensor with 1 cm strip length, 500 $\mu$m strip pitch, and 50 $\mu$m metalized electrodes.
• Figure 3: The simplified RC-network model for the AC-LGAD strip-sensor. The strip pads for electrodes 5, 6 and 7 are shown. The GND BIAS is the bias applied to the $n^+$ resistive layer of Fig. \ref{['fig:sensor_xsec']}. The $C_{\rm strip}$ is the strip capacitance to the backside (HV BIAS), and $C_{\rm couple}$ is the coupling capacitance from the strip to GND BIAS. The $C_{\rm couple}$ is significantly larger than $C_{\rm strip}$ to ensure that most of the charge is collected on the pad.
• Figure 4: A schematic diagram of the capacitive divider measuring the capacitance $C_{\rm in}$ of a single strip.
• Figure 5: The schematic diagram of the extracted model for the 1-cm long AC-LGAD strip sensor.