“Noise” can have different meanings. It could be the common phenomenon of, say, a buzz in an audio system. Other times it may refer to something less acoustic, perhaps a limit on the precision of measurements. As an example of the way the latter has become more problematic for designers, consider the analog portion of one channel in an industrial control or automotive system.
As IC and sensor supply voltages keep shrinking, that kind of noise has become increasingly troublesome. Employing ±22-V operating voltages used to be common, but now we see ±1.5 or even ±0.9 V. At the same time, applications need greater precision and accuracy. Many apps have moved from 8 bits to 12 and higher. These trends make measurements of microvolts challenging.
For example, for a 14-bit system, when full-scale was 5 V, the least significant bit (LSB) represented 305 µV. Now, for a realworld signal of 30 mV full-scale, at 12 bits (don’t even think about 14), half an LSB is 3.5 µV. In that kind of situation, if there were just 1 µV of input-referred error or noise from the amplifier, the measurement would be invalid.
MEASUREMENT NOISE
If you want to know all of the fundamentals about signal-conditioning amplifiers, including their noise performance, you can’t go wrong with Analog Devices’ Op Amp Applications Handbook (2006), edited by Walt Jung. It can be downloaded as a .pdf file at http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html.
Much of what follows is distilled from that, with further input from ADI’s Reza Moghimi. Moghimi also has a pair of webinars on intrinsic and extrinsic noise that can be accessed at www.analog.com/webinar/noise-optimization1and2.
All ICs contain inherent noise sources. In amplifiers, they can be modeled as zeroimpedance voltage generators in series with the input (en) and infinite-impedance current sources in parallel with the input (in). (The lower-case convention for potential and current indicates noise spectral density that’s a quantity that varies across frequencies. Upper-case indicates instantaneous values at specific frequencies.)
The noise from these intrinsic sources has different characteristics, depending on how it arises. Some of the terminology is fanciful. There’s white and pink noise, popcorn noise, shot noise, avalanche noise, and thermal noise. (While those are the most common designations, alternative terms will often be encountered.)
Other characteristics also are derived from noise. For instance, an amplifier’s noise figure (expressed in dB) is the amount by which the amplifier’s noise exceeds the noise of a perfect amplifier in the same environment. It’s generally only used in communications work.
Critically, the noise floor of the systems and a limiting factor for system resolution is the white or broadband noise. Observed in the frequency domain, it’s the flat part of the circuit’s noise spectrum. In expressing it, bandwidth must be specified. If F is frequency:
That is, it can be approximated as simply en times the square root of the upper frequency limit.
Distinguished from white noise, pink noise (also called flicker, or 1/f noise) occurs below a certain value called the corner frequency (FC). In that lower region, it increases inversely with frequency at 3 dB/octave (Fig. 1). (Actually, there’s no hard corner. The transition occurs gradually. Corner frequency is determined by extending the straight-line portions of white and pink noise and noting where they cross.)
Pink noise only occurs under conditions where current is flowing. It’s a manifestation of charge carriers being captured and released randomly. In bipolar transistors, that’s due to contamination and imperfect surface conditions at the base-emitter junction. In CMOS devices, it’s primarily associated with extra electron energy states at the boundary between silicon and silicon dioxide.
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