An application report on op-amp noise analysis from Texas Instruments. Link here (PDF)
As system bandwidths have increased, an accurate estimate of the noise contribution for each element in the signal channel has become increasingly important. Many designers are not, however, particularly comfortable with the calculations required to predict the total noise for an op amp, or in the conversions between the different descriptions of noise. Considerable inconsistency between manufacturers in describing noise and, in some cases, incomplete specifications, have contributed to this confusion. A thorough description of the op amp noise model will be developed here with a detailed discussion of the key differences between current and voltage feedback amplifiers. The conversions between several different measures for noise used in the industry will also be described. Broadband effects will be covered for both low frequencies (the 1/f region) and high frequencies (noise power bandwidth).