AMS Airport
 

Flying back to States from Frankfurt with stop/change planes in Amsterdam (KLM to Cont'l). Layover is 1 hr 40 mins. Is this sufficient?

yup if plane is on time...the transfer terminal area is not all that large and you do not have to go thru security again. You'll probably have time to shop at the many many stores in the transfer and international arrival and departures terminal.

You will go through immigration, and then right at your gate, security. But 1:40 should be enough.

You will NOT go through immigration, since you come from a "schengen" country. Time will be enough, hopefully you can enjoy the many taxfree shops.

The minimum connecting time is 50mins in AMS.

He's going from a Schengen country to AMS, then to a non-Schengen country (US). Therefore the OP needs to go through immigration to exit Schengen.