Active filters are another common application for op amps.
For active filters, especially in the low-pass configuration, you want to have substantially more bandwidth than your maximum signal frequency. This is because in an active filter application, you are actually getting band reject characteristics from the gain of the amplifier. So over the area that you need maximum attenuation in the stop band, you also need to have closed loop bandwidth in your amplifier.
- For high-pass filters, you want to make sure that your op amp has gain over the signal frequencies that you are interested in passing. The low frequency signals will be attenuated based on the closed loop gain of your part, but you don't want your part rolling off in the middle of your pass band.
- You want to look for good distortion: second and third harmonic. Intermodulation products are related to the third order distortion products and those will typically fall in-band if you have multiple signals. Normally, active filters are run at a low gain. High gain in active filters is typically achieved through multiple stages, so you are looking at a gain range of around one to ten.
- With current feedback op amps, you need to be very careful about the topology that you choose. You canot have parallel resistance with your feedback, or parallel capacitance with your feedback resistor.
- One of the popular filter topologies for cureent feedback op amps is the Sallen-Key filter.
- This schematic is for a second order filter. If you want a single order filter you can cascade a single order RC stage or a single pole filter before or after this. Notice that there is no capacitor anywhere near the feedback resistor on this topology. That makes it extremely suitable for a current feedback op amp.
It's True! The following National Semiconductor parts are ideal for these types of applications: LMH6702 & LMH6732
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Tip National Semiconductor offers a very informative tutorial on filters. It is Fundamentals of Active Filters
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