  We present the design and performance characterization results of the novel
Fermilab Constant Fraction Discriminator ASIC (FCFD) developed to readout low
gain avalanche detector (LGAD) signals by directly using a constant fraction
discriminator (CFD) to measure signal arrival time. Silicon detectors with time
resolutions less than 30 ps will play a critical role in future collider
experiments, and LGADs have been demonstrated to provide the required time
resolution and radiation tolerance for many such applications. The FCFD has a
specially designed discriminator that is robust against amplitude variations of
the signal from the LGAD that normally requires an additional correction step
when using a traditional leading edge discriminator based measurement. The
application of the CFD directly in the ASIC promises to be more reliable and
reduces the complication of timing detectors during their operation. We will
present a summary of the measured performance of the FCFD for input signals
generated by internal charge injection, LGAD signals from an infrared laser,
and LGAD signals from minimum-ionizing particles. The mean time response for a
wide range of LGAD signal amplitudes has been measured to vary no more than 15
ps, orders of magnitude more stable than an uncorrected leading edge
discriminator based measurement, and effectively removes the need for any
additional time-walk correction. The measured contribution to the time
resolution from the FCFD ASIC is also found to be 10 ps for signals with charge
above 20 fC.
