How To Design Active Filter

How Not to Design Active Filters

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QUESTION:

Why does my carefully designed active filter non meet its specifications?

Reply:

Considering much active filter software ignores 'existent life' amplifier behavior. I was recently on holiday diving in the Blood-red Sea. I had left my jail cell phone and computer at dwelling and forgotten all virtually work. But, foolishly, I carried my towel and camera to the dive boat in an Analog Devices rucksack. 1 of my fellow divers, Ekaterin from Russia, had just designed an active filter with ADI op amps, was having some problems, and recognized the logo. So much for a full break!

Her filter design was flawless. Spice assay confirmed it, and the components were properly toleranced. Luckily, I did not demand a calculator to come across why the filter did non worked as expected. The design had been done with an "platonic" op amp where all the parameters are either zero or infinity. Real life is rarely so all-around.

Voltage feedback op amps typically accept high open-loop proceeds and a unmarried-pole frequency response. High precision types have gain >10⁶ but their gain-bandwidth product is rarely more than a few MHz, so their open up-loop gain starts to drop at a few Hz. By 20 kHz, the top of the audio spectrum, the open-loop gain of a precision op amp may exist < 50—low enough to dethrone an active filter design. Furthermore at high signal levels, slew charge per unit also limits an amplifier'south frequency response

High-speed op amps exercise not have these problems, just many fast op amps are current feedback types which oscillate with capacitive feedback. Since many agile filter topologies apply capacitive feedback information technology is unwise to design active filters with them.

Designers often utilise high values resistors in order to utilize small-scale, cheap precision capacitors. Bias currents flowing in loftier resistances will dethrone the amplifier'southward offset voltage by the voltage drop in the resistance. The op amp's noise electric current will also make a greater contribution to system racket.

The resistor'south (Johnson) noise tin as well exceed the op amp dissonance. Not all filter designers consider this, nor do they always recall to provide the proper high frequency supply decoupling, thus impairing the amplifier's loftier frequency response.

Ekaterin'southward problem was due to the apply of too slow an amplifier and, luckily, I was able to recommend a faster one. This, every bit I learned shortly afterward my return home, allowed her circuit to exceed its required performance comfortably. After solving the active filter problem, we both returned to the water, and the beautiful reef life, with no more than distracting thoughts of piece of work.

How Non to Pattern Active Filters (ppt)

Author

James Bryant

James Bryant was a European applications manager at Analog Devices from 1982 to his retirement in 2009 and he still writes and consults for the visitor. He holds a degree in physics and philosophy from the University of Leeds and is also C.Eng., EurEng., MIET, and an FBIS. In improver to his passion for engineering, James is a radio ham and holds the phone call sign G4CLF.