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Selecting analogue integrated circuits (Part 1)

12 Nov 2014  | James Bryant

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Benjamin Franklin, a great scientist (and lucky—he flew a kite in a thunderstorm and survived to tell of it), once said that "In this world nothing is certain but death and taxes." But any experienced electronic engineer will tell you that "In this world nothing is certain but noise, death and taxes." Noise, an unwanted signal in a circuit or system, is everywhere—and minimising noise is an important part of circuit and system design.

There are two types of noise:
 • Signals which are wanted by you or someone else—but which have leaked to a place where they are not wanted, and
 • Unwanted signals generated by fundamental physical processes within electronic components.

The first sort is not a topic for this article—but when designing with analogue ICs it is important to protect them, by screening, decoupling and similar techniques, from exposure to external noise sources which might degrade their performance.

When choosing analogue ICs it is important to review their noise specifications and ensure that the noise generated by the device will not have too great an effect on system performance. Remember that any analogue input has both voltage noise and current noise, and these are, largely, uncorrelated—in most low-impedance circuits the current noise can usually be ignored, but in high impedance circuits the current noise, flowing in the high impedance, causes a noise voltage which must be considered when choosing a low-noise analogue circuit.[5] (If the high impedance is a high resistance, rather than a high reactive impedance, it is also important to consider the thermal, or Johnson, noise of the resistance itself.)

When selecting an analogue IC it is important to check its specification to ensure that its internal noise, in the circuit configuration that you are using, is not so large as to compromise the required system performance.


Smaller packages

Figure 4: Smaller packages.


The choice of package for an IC will usually be governed by considerations of available assembly technology, and possibly of the packages used for other ICs in the system. In general smaller packages are cheaper, not only in themselves but in terms of printed circuit board (PCB) cost, which is usually proportional to PCB size.

It is tempting to assume that the package is unimportant and that the characteristics of the chip in it are paramount—but in practice the choice of package may influence how easily we may use the chip to best advantage. It is important to give some thought on the issues involved.

It is obvious that IC packages are shrinking year by year—as with Moore's Law there is a physical limit to how long this can continue but we can expect the trend to continue for a while yet. Smaller packages have both advantages and disadvantages—they may be both cheaper in themselves and cheaper to use, they usually have smaller parasitic reactances (inductance and capacitance) and so perform better at high frequencies; and often but not always—read the data sheet carefully—they have lower thermal resistance, θja, and so may be used with higher power levels! This is counter-intuitive—we might expect smaller packages to get hotter—but arises from the fact that the most direct thermal path is often through the leads and smaller packages have shorter leads.

A serious problem with new packages is that there are often several packages with very similar names (even from the same manufacturer) and different PCB footprints, making it very easy to design a PCB which will not hold the IC that you thought it was designed for. Occasionally data sheet writers become confused and, although this is usually corrected very quickly, it is not completely unknown for the wrong package to be shown on a data sheet! It is therefore extremely important to verify that PCBs are designed with the correct footprints for the ICs that they are to carry—and to use the most up-to-date data sheets.

A final package issue is that small surface-mount packages are hard to handle when building laboratory prototypes [6]. Prototypes are critical when designing analogue circuits—software models allow efficient optimisation but cannot model all the peculiarities of a real-life IC. It is essential to build a prototype and ensure that its behaviour is congruent with that of the computer model [7].


References
[1] For over a quarter of a century I have been offering a bottle of whisky to anyone who can show me an Analog Devices data sheet with no mistakes whatsoever. Each year, on New Year's Day, I open last year's bottle for my own consumption—and I have not missed a year yet.

[2] This article applies to application specific integrated circuits (ASICs) as to any other type of IC, but we shall not consider any special characteristics or parameters of ASICs.

[3] RAQs 19, 31, and 50 all discuss various aspects of absolute maximum ratings:-

http://www.analog.com/static/imported-files/rarely_asked_questions/RAQ_maximumRatings.pdf

http://www.analog.com/static/imported-files/rarely_asked_questions/RAQ_tempRanges.pdf

http://www.analog.com/en/analog-to-digital-converters/ad-converters/products/raq_ja_whats_deal_about_absolute_max_ratings_iss50/resources/faq.html?display=popup

[4] RAQ 31 reminds us that there are some exceptions to absolute maximum rules where temperature is concerned:-

http://www.analog.com/static/imported-files/rarely_asked_questions/RAQ_tempRanges.pdf

[5] For more details about noise see RAQs 25 & 26 and the articles associated with them:

http://www.analog.com/en/high-speed-op-amps/low-noise-low-distortion-amplifiers/products/raq_jb_resistor_noise_can_be_deafening_issue25/resources/faq.html?display=popup

http://www.analog.com/en/all-operational-amplifiers-op-amps/operational-amplifiers-op-amps/products/RAQ_JB_Op_Amp_Noise_can_be_Deafening_Too_Issue26/resources/faq.html?display=popup

http://www.analog.com/static/imported-files/rarely_asked_questions/moreInfo_raq_opAmpNoise2.html

[6] Again—on breadboarding with SMDs RAQ 21 and its associated articles are your friends:-

http://www.analog.com/en/analog-to-digital-converters/ad-converters/products/raq_jb_breadboarding_with_surface_mount_ic_issue21/resources/faq.html?display=popup

[7] And RAQ 61 has more thoughts on prototyping:-

http://www.analog.com/en/all-operational-amplifiers-op-amps/operational-amplifiers-op-amps/products/RAQ_Make_Do_Issue61/resources/faq.html?display=popup


About the author
James Bryant was European Applications Manager for Analog Devices for over a quarter of a century, and is a well-known lecturer on analogue electronics. He is the author of many articles, papers and application notes on analogue electronics, of many of Analog Devices'Technical Seminars, and of about a third of their monthly"Rarely Asked Questions"columns.


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