Op-amp noise is calculated from the voltage and current spectral densities provided by the user on the MODEL record. These parameters are often available from the manufacturer's data sheets. The corner frequencies for voltage and current 1/f noise can also often be obtained or at least estimated from data sheets. The voltage spectral density source is added at the input to the op-amp, effectively in series with the non-inverting input terminal. The current spectral density source is added at each input terminal to ground.