How do I calculate the junction temperature of the AD8603?
Tj = Ta + ( Pd * THETAja)
Tj = junction temperature
Ta = ambient temperature
Pd = total power dissipation
THETAja = junction-ambient thermal resistance
Thermal resistance is highly package dependent. Please look up the Thermal Resistance table (Table 4) that we include in every op amp data sheet. Also, make sure that you do not violate any temperature range in the absolute maximum ratings table (Table 3).
MT-093 explains some thermal design basics and provides very useful information.