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When selecting an op amp for any given project there are a number of choices to be made. Some op amps are general purpose ones and suit many general purpose applications, whereas other op amps have been designed with higher levels of performance in some areas.
When designing a circuit using op amps, it is necessary to choose the right type to ensure that the best performance is obtained for the best cost.
In many circuits the standard general purpose op-amps will perform very well and the higher performance levels of specialist op amps will not be needed, but in some applications enhanced performance is required and the more specialist op amps can be chosen.
Op amp requirements
The ideal op amp, should it ever exist, would have infinite gain, infinite input impedance, zero output impedance, it should have an infinite frequency response, not introduce any noise, and it should be distortion free. Obviously no op amp can meet all these requirements.
However when selecting the right op amp there is a balance to be made between the different op amps. General purpose ones will perform adequately in most areas, whereas, some others may have a very high input impedance, or low noise, or wide frequency response.
When designing an op amp circuit, it is necessary to know what is required, so that the best op amp for the job can be chosen.
High input impedance
In some applications a very high input impedance op amp may need to be chosen. A number of instances may arise where a very much higher impedance is required than that normally available on a general purpose op amp. On example may be for an integrator. When the charge is integrated over a long period of time, even the high impedance of a standard op amp will be noticed as charge will leak away.
There may also be instances where high impedance probes are used, and an op amp placed very close to the probe as a buffer to prevent stay pick-up. Here a high impedance op amp may be needed. There are also many other applications where high impedance op amps may be required.
In instances like these op amps with FET inputs may need to be selected. These op amps present exceedingly high levels of impedance at the input. A current path for biasing the input still needs to be incorporated as the FETs need to be properly biased, but even so these FET input op amps provide an excellent choice.
Another parameter of importance in some applications is the noise. Where the circuits demand a good noise performance it is necessary to choose a low noise op amp. These devices are readily available as there are many areas where selecting a low noise device is important.
Noise can be particularly important in the input stages of a product. Further stages of amplification will only amplify the noise and it cannot be removed.
If using the op amp for an input stage, then it may well be advisable to choose a low noise op amp.
Low power / current
With many items requiring to be battery powered these days, low power consumption can be an issue. Many op amps have been designed for these applications, and by searching, it is possible to choose some very low power op amps.
Low supply voltage
Early op amps used to run from supplies that might be ±15V. Nowadays with many circuits needing to run from much lower supplies these voltages are not practicable in many instances. Fortunately it is possible to choose low voltage op amps.
Choosing the right op amp package
When choosing an op amp, it is also necessary to select the package type. The chips can be obtained in a variety of different packages, both conventional though hole mounting in 4 and 8 pin dual in line as well as a variety of surface mount packages as well.
Also don’t forget that it is possible to select packages with multiple op amps within them. Typically they can come with two and four op amps per package, although higher counts are available. These can save space on the board and cost . When opting for the higher number op amps per package, remember these multiple devices per package do not normally come with offset null connections. Check this before choosing the device if offset null is required.
Choosing an op-amp that matches the needs of any given application, first needs the requirements to be understood. Once this has been done, the requirements can be compared to the devices that are available and the best choice of op amp can be made.
Short circuiting the process is likely to mean that the wrong device is chosen. Following a logical approach means that the right op amp is chosen to meet the needs for the circuit in question.