The group delay is for a digital filter is defined as the time (i.e. # of
samples) it takes for the output of the digital filter to reach its peak when
excited by an impulse. To determine the group delay in terms of the # of
samples, multiply the Absolute Group Delay spec (in usec) by the sample rate
(i.e. 20 MHz) on page 5 of the datasheet. For example, the group delay for 8X
mode is 13.55 usec x 20 MHz=271. Note, this corresponds to figure 1b on page 6
of the datasheet.