previous Power Supplies and Decoupling
This section examines in more detail some of the issues relating to amplifiers for use in precision signal conditioning applications. Although the OP177 op amp is used for the "gold standard" for precision in these discussions, more recent product introductions such as the rail-to-rail output OP777, OP727, and OP747, along with the OP1177, OP2177, and OP4177 offer nearly as good performance in smaller packages.
Precision op amp open-loop gains greater than 1 million are available, along with common-mode and power supply rejection ratios of the same magnitude. Offset voltages of less than 25μV and offset drift less than 0.1μV/°C are available in dual supply op amps such as the OP177, however, the performance in single-supply precision bipolar op amps may sometimes fall short of this performance. This is the tradeoff that must sometimes be made in low power, low voltage applications. On the other hand, however, modern chopper stabilized op amps provide offsets and offset voltage drifts which cannot be distinguished from noise, and these devices operate on single supplies and provide rail-to-rail inputs and outputs. They too come with their own set of problems that are discussed later within this section.
It is important to understand that DC open-loop gain, offset voltage, power supply rejection (PSR), and common-mode rejection (CMR) alone shouldn't be the only considerations in selecting precision amplifiers. The AC performance of the amplifier is also important, even at "low" frequencies. Open-loop gain, PSR, and CMR all have relatively low corner frequencies, and therefore what may be considered "low" frequency may actually fall above these corner frequencies, increasing errors above the value predicted solely by the DC parameters. For example, an amplifier having a DC open-loop gain of 10 million and a unity-gain crossover frequency of 1MHz has a corresponding corner frequency of 0.1Hz! One must therefore consider the open-loop gain at the actual signal frequency. The relationship between the single-pole unity-gain crossover frequency, fu, the signal frequency, fsig, and the open-loop gain AVOL(fsig) (measured at the signal frequency is given by:
It the example above, the open-loop gain is 10 at 100kHz, and 100,000 at 10Hz. Note that the constant gain-bandwidth product concept only holds true for VFB op amps. It doesn't apply to CFB op amps, but then they are rarely used in precision applications.
Loss of open-loop gain at the frequency of interest can introduce distortion, especially at audio frequencies. Loss of CMR or PSR at the line frequency or harmonics thereof can also introduce errors.
The challenge of selecting the right amplifier for a particular signal conditioning application has been complicated by the sheer proliferation of various types of amplifiers in various processes (Bipolar, Complementary Bipolar, BiFET, CMOS, BiCMOS, etc.) and architectures (traditional op amps, instrumentation amplifiers, chopper amplifiers, isolation amplifiers, etc.)
In addition, a wide selection of precision amplifiers are now available which operate on single-supply voltages which complicates the design process even further because of the reduced signal swings and voltage input and output restrictions. Offset voltage and noise are now a more significant portion of the input signal.
Precision op amp characteristics
Figure 1-87: Precision op amp characteristics
Selection guides and parametric search engines, which can simplify this process somewhat, are available on the world-wide-web (http://www.analog.com) as well as on CDROM. Some general attributes of precision op amps are summarized in Figure 1-87.
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