8.6.1 Noise

Random noise is generated by every real-world circuit. It can be generated by thermal noise in the case of resistors, 1/ f noise in the case of CMOS transistors, or quantization noise in the case of DACs and ADCs. Noise can also be injected into a circuit by external forces, such as light falling on a bare die or electromagnetic interference coupling into a circuit under test. Excessive noise can result in a hissing noise in audio circuits, corrupted data in a modem or cellular telephone, and many other system-level failure mechanisms. Noise is generally, but not always, an undesirable property of a circuit under test. Noise is one of the leading causes of long test time, since averaging or added measurements are needed to remove the nonrepeatability caused by random noise.

The spectral density of noise energy is often described using color analogies. White noise, like white light, contains energy that is evenly distributed across the frequency spectrum. White noise is noise whose RMS voltage is constant in any band of frequencies from F to F+ ? F, regardless of the value of F. Pink noise, by contrast, is noise that is weighted more heavily at low frequencies.

Often the level of noise is assumed to exhibit a Guassian (normal) statistical distribution. This is largely a result of the central limit theorem of large numbers. It is important to recognize, however, that the spectral properties of noise and its...