Time to get started! The ADA2200 Data Sheet and the ADA2200SDP-EVALZ-UG-787 explain the part I plan to use and show an evaluation board design that I will use for guidance. In addition, the design in https://hackaday.io/project/12491-spectrophotometer will be used especially for the output connection to the Teensy. The Teensy is not required. Other control schemes could easily be used.

To aid understanding, the Data Sheet has a good explanation of the operation of a Lock In Amplifier. The Orozco article (lock_in_amplifier.pdf) has another good explanation. Together, they should provide a basic understanding. There are many other papers that offer more academic and mathematical explanations, but these can be read later.