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Functionality Differences of Dual-Supply and Single-Supply Devices

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There are two major classes of op amps, the choice of which determines how well the selected part will function in a given system. Traditionally, many op amps have been designed to operate on a dual power supply system, which has typically been ±15V. This custom has been prevalent since the earliest IC op amps days, dating back to the mid-sixties. Such devices can accommodate input/output ranges of ±10V (or slightly more), but when operated on supplies of appreciably lower voltage, for example ±5V or less, they suffer either loss of performance, or simply don’t operate at all. This type of device is referenced here as a dual-supply op amp design. This moniker indicates that it performs optimally on dual voltage systems only, typically ±15V. It may or may not also work at appreciably lower voltages.
Comparison of relative functional performance differences between single and dual-supply op amps
Figure 1-10: Comparison of relative functional performance differences between single and dual-supply op amps
Figure 1-10 above illustrates in a broad overview the relative functional performance differences that distinguish the dual-supply vs. single-supply op amp classes. This table is arranged to illustrate various general performance parameters, with an emphasis on the contrast between single and dual-supply devices. Which particular performance area is more critical will determine what type of device will be the better system choice.
More recently, with increasing design attention to lower overall system power and the use of single rail power, the single-supply op amp has come into vogue. This has not been without good reason, as the virtues of using single supply rails can be quite compelling. A review of Fig. 1-10 illustrates key points of the dual vs. single supply op amp question.
In terms of supply voltage limitations, there is a crossover region in terms of overall utility, which occurs around 10V of total supply voltage.
For example, single-supply devices tend to excel in terms of their input and output voltage dynamic ranges. Note that in Figure 1-10 a maximum range is stated as a % of available supply. Single-supply parts operate better in this regard, because they are internally designed to maximize these respective ranges. For example, it is not unusual for a device operating from 5V to swing 4.8V at the output, and so on.
But, rather interestingly, such devices are also usually restricted to lower supply ranges (only), so their upper dynamic range in absolute terms is actually more limited. For example, a traditional ±15V dual-supply device can typically swing 20Vp-p, or more than four times that of a 5V single-supply part. If the total dynamic range is considered (assuming an identical input noise), the dual-supply operated part will have 4 times (or 12 dB) greater dynamic range than that of the 5V operated part. Or, stated in another way, the input errors of a real part such as noise, drift etc., become 4 times more critical (relatively speaking), when the output dynamic range is reduced by a factor of 4. Note that these comparisons do not involve any actual device specifications, they are simply system-based observations. Device specifications are covered later in this chapter.
In terms of total voltage and current output, dual-supply parts tend to offer more in absolute terms, since single-supply parts usually are usually designed not just for low operating voltage ranges, but also more modest current outputs.
In terms of precision, the dual-supply op amp has been long favored by designers for highest overall precision. However, this status quo is now beginning to be challenged, by such single-supply parts as the truly excellent chopper-stabilized op amps. With more and more new op amps being designed for single-supply use, high precision is likely to become an ever-increasing strength of this category.
Load immunity is often an application problem with single-supply parts, as many of them use common-emitter or common-source output stages, to maximize signal swing. Such stages are typically much more load sensitive than the classic common-collector stages generally used in dual-supply op amps.
There are now a greater variety of dual-supply op amps available. However, this is at least in part due to the ~30-year head start they have been enjoying. Currently, new op amp designs are increasingly oriented around one or more aspects of single-supply compatibility, with strong trends toward lower supply voltages, smaller packages, etc.
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