Op Amp Applications Handbook Walt Jung, Editor

19
Op Amp Basics
dual- supply device can typically swing 20 V p-p, or more than four times that of a 5 V single-supply part.
If the total dynamic range is considered (assuming an identical input noise), the dual-supply operated
part will have four times (or 12 dB) greater dynamic range than that of the 5 V operated part. Or, stated in
another way, the input errors of a real part such as noise, drift, and so forth, become four 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 specifi cations, they are simply system-based observations. Device
specifi cations 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 are usually designed not just for low operating voltage ranges, but also for more modest
current outputs.
In terms of precision, the dual-supply op amp has long been 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 commonemitter
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 is 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, and so forth.
Device Selection Drivers
As the op amp design process is begun, it is useful to keep in mind the fact that there are several selection
drivers, which can dictate priorities. This is illustrated by Figure 1-11.
FUNCTION PERFORM PACKAGE MARKET
Single, Dual,
Quad
Single Or
Dual Supply
Supply
Voltage
Precision Type Cost
Speed
Distortion,
Noise
Low Bias
Current
Power
Size Availability
Footprint
Figure 1-11: Some op amp selection drivers
Actually, any single heading along the top of this chart can, in fact, be the dominant selection driver and
take precedence over all of the others. In the early days of op amp design, when such things as supply
range, package type, and so forth, were fairly narrow in spread, performance was usually the major driver.
Of course, it is still very much so and will always be. But, today’s systems are much more compact and
lower in power, so things like package type, size, supply range, and multiple devices can often be major
drivers of selection. As one example, if the only available supply voltage is 3 V, look at 3 V compatible
devices fi rst, and then fi ll other performance parameters as you can.