Glenn,

Read the app. note as it is quite informative, good link Rob. However, keep in mind that the input structure of the '2051/'4051 p0.0 and p0.1 which are different from the Cygnal part.

Specifically this regards transitions thru the linear region between what otherwise would be valid low or high. You can do things on these pins you would never do on the others.

Unfortunately, I haven't found great detail on these pins and had to test them myself.

A note of interest may be:
A/D conversion

regards,
p