Noise of resistors is divided into thermal noise and current noise. Thermal noise is logically determined by resistance, absolute temperature and frequency range, and has no relation with resistor material. The effective value in unit interval of frequency is extremely small and distributed uniformly in frequency domain, which leads to main factor of deterioration of S/N in broad band amplifier of wireless frequency. Normally, no problem occurs in low frequency range under audio frequency. The current noise, on the contrary, depends much on material, the lower the frequency, the larger the noise becomes. In the frequency range under kHz, the effective value of unit interval of frequency is larger than thermal noise; especially under a few Hz, it increases incomparably. Optical detecting circuit as below which amplifies micro detecting signal with large gain, or applications like elctrocardiographic monitors which handle micro voltage near direct current, current noise rather than thermal noise would be the main factor of deterioration of S/N. In these purposes, thin film chip resistors with excellent current noise characteristics like RN73R, RN73H, MF, RNS, MRS are suitable.
Example of circuit easily influenced from current noise(optical detecting circuit)