how to measure more than 500°C on the electronics ?

Hello guy`s

I have to measure at more than 500°C but running into a issue.
My setup is:
K-Type Thermocouple up to 800 °C
Sanguinololu Board (also can test megatronics,gen6delux, tensyluu), connected the Thermo-board to Pin 4
Thermocouple Preamplifier board with AD597 from Reprapworld.com connected 12V to +
Marlin thermistor is set -1 for the Thermocouple, pin is also assigned correctly.

So now i can measure from 0°C to 499,5°C thats where the AD597 produces 5.0V output to analog Pin 4 of the Sanguino.
althouth when i heat the element more up there comes more voltage out of it (up to about 5.4 volts with a lighter)

So for my understanding, do all electronics only measure between 0 and 5volts ?
So if i want to measure higher do i have to put in a "offset" resistor (what value?) to lower the voltage and then change the firmware, but what would i have to change ?
I think this may work to get a reading at higher temperatures then so for eg. from 300 to 800°c but it`s not a very elegant solution.
Is there maybe a other way to solve this to measure the full range from 0 to 800°C ?

Chri