Sunday, 29 January 2012

Operational Amplifier Noise: Techniques and Tips for Analyzing and Reducing Noise

Operational Amplifier Noise: Techniques and Tips for Analyzing and Reducing Noise

by Paul McGoldrick

Operational Amplifier Noise: Techniques and Tips for Analyzing and Reducing Noise
by Art Kay, Published by Newnes (Elsevier)
ISBN 13: 978-0-7506-8525-2, hardback, 238 pp, $69.95, January 2012

Over the decades books about op amps have followed very similar paths and they have, in general, been far too academic in their approach. As someone who was baffled in college by the notion of virtual grounds in an approach to signal path analysis, I find it wonderful to see these circuits reduced down to the simple blocks of TI’s TINA SPICE interpretation and understanding.
Art Kay is a twenty-year veteran of analog engineering and earned an MSEE from Georgia Tech. He is currently the linear applications manager for TI in Tucson and specializes in sensor signal conditioning; he has also focused on noise and this text is a unique view of what most of us thought was too complex an area for us to properly spend the time to understand. Art leads us through examples and analysis. Although there is plenty of mathematics, the majority should not bug the graduate engineer.
Each of the eleven chapters of Operational Amplifier Noise offers us a lot of visuals as we get to see what is what and, like an illustrated book, doesn’t absolutely need us to read the text – although we would be a little daft if we didn’t. The author then summarizes the chapter and poses a few questions to make sure the messages got home.
Chapter 1 introduces noise in both time and frequency domains as well as looking at how noise is a statistical Gaussian fact as well. We are then taught how to correctly integrate spectral density (as a power) into rms noise voltage so we can understand those data sheet numbers a little better.
Chapter 2 really starts into the op amp world and current noise, voltage noise, and thermal noise together with the 1/f noise region while Chapter 3 gets into the meat of calculating noise examples. That hand analysis is then compared, in Chapter 4, to the results from the superb TINA SPICE modeling although other models can, of course, be used if another analysis program is employed.
Chapter 5 takes us into the world of practical noise measurements so we have something to compare to our hand-worked or program analyses of noise. True rms metering leads into scope displays of noise (allowing us to see more of the true nature of the unwanted signals) and then into frequency-domain displays with a spectrum analyzer. Shielding of measurements is emphasized and shown by example.
Noise inside the op amp is looked at in more detail in Chapter 6 and there are five wonderful rules of thumb for worst case noise analysis. FET noise is also discussed while Chapter 7 looks at the wonderful low-frequency world of popcorn noise and how its non-predictability makes it all the more fascinating – especially as it is considered enough to render a device as a failure.
Zero-drift amplifiers (an ideal, of course, that is never actually met in practice) do not have 1/f noise during the calibration process but at a cost with frequency spikes – and often harmonics – in the spectral density curve. Chapter 8 describes these effects in great detail and also offers the very practical reminder that temperature drift can look like 1/f noise. Standard deviation measurements of noise are also emphasized – rms measurements have a dc component, which do not belong in noise.
Chapter 9 builds on the op amp lessons by incorporating three of them and referring all the noise back to the input in a typical instrumentation amplifier arrangement and analyzing them by hand, simulation and measurement.
Chapter 10 specifically addresses photodiode amplifier noise with emphasis on the transimpedance amplifier while Chapter 11 wraps it up with examples in the usual calculations, simulations, and practical formats we have been led to expect throughout the text. The practical world of measurement is also covered when it is necessary to use a post-amplifier to lift the noise being measured well above the noise floor of the test equipment. The author emphasizes how the input noise of the post-amplification is minimal to the actual noise measurement.
The book is finished off with a glossary that also includes figures – a really unusual format – answers to the chapter questions that have been posed, and a fairly complete index.
Operational Amplifier Noise is not a low-cost textbook but that will not stop the buyer who really wants to understand and do something about noise in his designs. There are negatives, of course, and they are mainly centered around some of the editing that has taken place, and some of the editing that should have occurred. The most heinous, in my opinion, is the extraordinary use of compound adjectives in units, such as “3.526-┬ÁV RMS (sic)” and the most noticeable is the unfortunate repeat of text on the back cover. The book is rich in figures and that makes layouts very difficult but it is well achieved for the most part although, by necessity perhaps, sizing has been a little arbitrary in some cases.
While it's been EN-Genius' privilege to host most of the book’s content on our site as TechNotes, we would be the first to say that just having the text in your hands in hardback form is a great pleasure.

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