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Analogue integrated circuits 169But nothing comes for free: the negative aspect of multi-sourcing is that manyparameters go unspecified for cheap devices, and this leaves open the possibility thatdifferent manufacturers’ nominally identical parts can differ substantially in thoseparameters that are omitted from the common spec. If you’ve designed and tested acircuit with manufacturer A’s devices, and they happen to be quite fast, you will beheading for production problems when your purchasing manager buys a few thousandof manufacturer B’s devices which are slower. For instance, TI’s data sheet for theLM324 gives a typical slew rate of 0.5V/µs at 5V supply; but National, who could fairlybe said to have invented the part, do not mention slew rate at all in theirs.To deal with this, design the circuit from the outset to be insensitive to thoseparameters which are badly specified, un-specified or (worse) specified differently inthe data sheets of each manufacturer. Or, look for a more tightly specified part.When not to use industry standardsWithin the last few years there has been a counter-trend to the imperative for multisourcingand the use of industry standards. Hundreds of new types have beenintroduced, and many of them are much better than their predecessors. They not onlyminimise the trade-offs between speed, power, precision and cost, but are also morefully specified. You can select a part by function and application – for instance, a DACbuffer or 75-ohm cable driver – rather than by comparing technologies, or by lookingat a particular specification such as gain bandwidth product. Following themanufacturer’s selection guides on the basis of application will often lead quickly to themost suitable part.Selecting more application specific ICs in this way steers the design process awayfrom industry standards. But there are a number of reasons why alternate sourcing hasbecome less of a necessity, despite its advantages given above. The average product lifecycle – sometimes months rather than years – is much shorter than the lifetime of a goodop-amp. In addition, qualifying multiple sources is a task that many designers don'thave time or expertise to do fully. Finally, for highly competitive products, you’ll haveto choose parts that give your design the edge (even if they are proprietary) and forwhich there may be nothing comparable in performance, cost, or functionality.Quad or dual packagesComparing prices, the LM324 does offer, in fact, the lowest cost-per-op-amp (5p). Thispoints up another factor to bear in mind when selecting devices: choose a quad or dualpackage in preference to a single device, when your circuit uses several gain stages.This reduces both unit cost and production cost. Such parts often have quiescent supplycurrents only slightly greater than a single-channel device, but with better offset,temperature tracking of drift, matching, and other specs. The disadvantages areinflexibility in supply voltage and pc layout, and possible thermal, power rail or RFinteraction between gain blocks on a single substrate.Some parts are available only in dual or quad configurations because single-channelversions would not have enough applications. Conversely, highest speed op amps, withbandwidths above several hundred megahertz, are often available in singles only,because of internal crosstalk. However, pinouts in multichannel configurations are lessstandardised than the basic single-channel unit, so substitutes are harder to find.5.2.16 Current feedback op-ampsThere are also op-amps that use current feedback topology instead of the more familiar
170 The Circuit Designer’s Companionvoltage feedback. Voltage feedback is the classic, well-understood mechanism whichwe have been discussing all the way through this section so far. In current feedback, theerror signal is a current flowing into the inverting input; the input buffer’s lowimpedance, in contrast to a voltage amplifier’s high input impedance, allows largecurrents to flow into it with negligible voltage offset. This current is the slewing current,and slew rate is a function of the feedback resistor and change in output voltage.Therefore, the current-feedback amplifier has nearly constant output transition times,regardless of amplitude.A very small change in current at the inverting input will cause a large change inoutput voltage. Instead of open-loop voltage gain, the current feedback op-amp ischaracterised by current gain or “transimpedance” Z S . As long as Z S >> R F , thefeedback resistor, the steady-state (non-slewing) current at the inverting input is smalland it is still possible to use the usual op-amp assumptions as initial approximations forcircuit analysis, i.e. the differential voltage between the inputs is negligible, as is thedifferential current.R GV VR FR GR F–V IN–Z SZ SV IN +OUT+OUTnon-inverting invertingV OUT = V IN · (1 + R F /R G ) · (1/(1 + R F /Z S )) V OUT = –V IN · (R F /R G ) · (1/(1 + R F /Z S ))Figure 5.16 The current feedback circuitIn performance, current feedback generally offers higher slew rate for a givenpower consumption than voltage feedback, and voltage feedback offers you flexibilityin selecting a feedback resistor, two high-impedance inputs, and better DCspecifications. With a current-feedback op-amp, you first set the desired bandwidth viathe feedback resistor, and then the gain is set according to the usual resistive ratios. Thismeans that the wider the bandwidth, the lower will be the operating impedances. If R Fis doubled, the bandwidth will be halved. The circuit becomes less stable whencapacitance is added across the feedback resistor.Current feedback devices tend to be used only at higher frequencies, forapplications such as professional video and high-performance widebandinstrumentation. The same part can be used in several applications for quantity costsavings, using only as much bandwidth as needed. They are less common in lower endconsumer applications because they need more design expertise. Current feedback is no“better” or “worse” than voltage, which is also capable of similar performance in theright design, but it does provide an alternative which is worth considering in theappropriate application.5.3 ComparatorsA comparator is just an op-amp with a faster slew rate, and with its output optimisedfor switching. It is intended to be used open-loop, so that feedback stabilityconsiderations don’t apply. The device exploits the very large open-loop gain of the op-
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The Circuit Designer’s Companion
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NewnesAn imprint of ElsevierLinacre
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vi Contents2.2 Design rules 452.2.1
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viii ContentsChapter 5Analogue inte
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x Contents7.5.3 Secondary cells 256
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xii Contents
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2 The Circuit Designer’s Companio
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10 The Circuit Designer’s Compani
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Grounding and wiring 25Cable type R
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Grounding and wiring 27Shielding an
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Grounding and wiring 29successive h
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Grounding and wiring 31(a) Two audi
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Grounding and wiring 331. Side-by-s
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Grounding and wiring 35ABV pZ out =
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Grounding and wiring 37times will d
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Grounding and wiring 39of Z o . Thu
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Printed circuits 41material. The co
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Printed circuits 43board cost you s
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Printed circuits 45Board ABoard ABo
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Printed circuits 47which they can w
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Table 3.3 Survey of capacitor types
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334 Indexde-rating 310dielectric ab
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336 IndexFlash ADC 209Foldback curr
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338 IndexMultilayer PCB constructio
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340 Indexlimiting element voltage 7
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