Noise that occurs in any electrical system as a purely physical phenomena can be defined as any undesirable voltage or current signal that interferes, distorts, or combines with the input or output of that system. Even if all external (extrinsic) noise sources are eliminated from coupling into a system (as the noise definition suggests), an internal (intrinsic) noise generates within the system. This intrinsic noise is present on the system output, and often, this intrinsic noise must be considered and measured. A simple system (device) like the LDO generates internal noise, which can be measured on the output. The noise of the LDO is a key specification in the datasheet, usually expressed over a frequency bandwidth (BW) of 10Hz–100kHz (typical). Further details on LDO noise can be found in the References [1] section. Graphs in the datasheet are often used to express the output noise of the LDO. Figure 1-1 shows the output noise for a typical LDO.

Any point on the noise curve is recognized as the spectral noise density of the LDO at that frequency. At low frequency, the noise of the LDO is dominated by flicker noise, then the noise rolls-off at the 1/f frequency to a lower noise level—thermal noise—then the curve of the LDO noise rolls-off due to the BW of the LDO rolling-off. The total noise, or RMS, is an additional metric reported in the electrical characteristics table for the LDO, accompanied by the specific frequency BW. This RMS noise is simply the accumulated, integrated, spectral-noise density under the noise curve over a specific BW, typically 10Hz–100kHz in units of μVRMS.