Analog Electronics for Measuring Systems

5Introduction to Noise Analysis in Low Frequency Circuits

Be not afeard; the isle is full of noises,Sounds and sweet airs, that give delight and hurt not.Sometimes a thousand twangling instrumentsWill hum about mine ears, and sometime voicesThat, if I then had waked after long sleep,Will make me sleep again: and then, in dreaming,The clouds methought would open and show richesReady to drop upon me that, when I waked,I cried to dream again.

William SHAKESPEARE, The Tempest

5.1. What is noise?

It is not easy to give a unique answer to that simple question. In fact, depending on the context, noise can be studied from a number of quite different points of view. However, the presence of noise constitutes one of the most important limitations for the performances of an analog circuit. No analysis on a measurement chain is therefore complete if it does not consider how noise affects performances. We will therefore adopt here a very pragmatic point of view, which will be enough for the purposes of an introductory book.

In the previous chapters, we followed the signal path from the sensor to the analog to digital converter. In our description, the signal was carrying useful information. However, along the processing chain, the signal might be distorted and some unwanted perturbations are inevitably added. It turns out that, at least in our cruel world, it is impossible to fabricate a circuit in which the signal is processed but not perturbed in any measurable way. A certain degree ...