First off, have you looked into a monitor driver IC for your specific display? Most displays like that have ICs for such purposes. Look up the datasheet for the display.
One way is to add an external ADC onto your current microcontroller board via either an I2C or SPI bus. Its not too difficult. Here are a couple of tutorials that should get you going.
First things first, you need to find out what bit-depth you need for your ADCs, 10-bit should probably do becasue its probably just a byte being transmitted at a time. You don't want an 8-bit ADC though if you are decoding 8-bit values because you will get errors. You need at least one bit extra because your LSb is going to have some noise in it.
Second you need to find out how many ADCs are on you microcontroller development board. Most common ones have 2-channel 10-bit ADCs multiplexed out through 16-pins through an analox MUX like the 74HC4051. If you need more than 8 extra channels, you can always chain together multiple ADCs.
If you want to find some 8-channel ADCs, you can find them on either Mouser and Digikey. Just go to their website, and navagate through their index to find the mutli-channel ADCs.
Digikey website > Product Index > Integrated Circuits (ICs) > Data Acquisition - Analog to Digital Converters (ADC)
mouser website > All Products > Semiconductors > Integrated Circuits - ICs > Data Converter ICs > ADC / DAC Multichannel