The signal bit (DB0) at pin 20 is toggling, when the /RDVEL is high (at the
same time sample by the signal /RD), while the resolver is rotating, and stays
at the last position when the resolver is stopped rotating.
But the same pin 20 (DB0) is toggling randomly when /RDVEL is low (at the same
time being sampled by the signal /RD), even when the resolver is stationary.
The AD2S1200's dynamic ripple specification is 1LSB typical and the overall
velocity accuracy specification that includes the dynamic ripple is 2 LSB max.
This means that a change in the LSB of the velocity output is highly possible
in your application.