ADuC7020 temperature sensor calculation


Is there an equation for calculating the the Celcius temperature from the output of ADCDAT?  The example calculation from the ADuC7020 datasheet page 50 is only accurate at room temperature.  As the temperature reaches 80C in the test chamber, the calculated temperature using the datasheet is about 20 degrees higher.  The equation I used is from the datasheet is:

floor((0x525 - (ADCDAT >> 16))/1.3)

Please show me the equation to use to calculate the temperature from ADCDAT.  Thanks.