This PLC Tutorial by http://www.bin95.com explains the analog controller using the A to D converter for PLC I/O. This is just a small, low resolution sample from DVD 6 of the 10 DVD PLC Training Video Course Library. The actually DVDs have much better quality and details. http://www.youtube.com/watch?v=rWMGY3CHaZU
For instructor based PLC Training, see https://www.youtube.com/watch?v=Bw-m9gT7Nh4