thermometer code to binary code
i'm not sure if i am understanding your intentions but... if i'm understood it right, you are mistaken in the use of the encoder...
an 8:3 encoder takes 8 inputs, and translates the active input to a digital number. This is, if you activate input number 0, you will get a 000 output. If you activate input number 5, you will get 110 in the output. The fact that it has priority means that when you already activated an input, it remains active even if you activate another one at the same time. This is, the first active input is the one that counts.
So, unless your ADC gives just 7 different values of reading (which i have never seen, and actually would be very unefficient), i don't see the purpose of connecting that encoder to the ADC...
i recommend you check the ADC's datasheet to see how it works