ECE3042 OA-26

ECE3042 OA-26 - Designing Active High Speed Filters...

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Designing Active High Speed Filters Filters built from resistors (R), inductors (L) and capacitors (C) are known as RLC or passive filters and are the domi- nant type of filter for high frequency applications. The per- formance of these filters is typically limited by the inductors which are large, expensive and far from ideal in an electrical sense. By using amplifiers and feedback, the same filter characteristics can be achieved without the use of inductors, these filters are known as “active filters”. An active filter will perform well only to the extent that the amplifiers in it behave in an ideal sense, so traditionally active filters have been limited to applications at frequencies below 1 MHz. With the advent of low cost, very high speed operational amplifiers, it is now feasible to realize active filters with frequency ranges in the tens of MHz. With some of the newer high speed amplifiers such as the CLC449 – a 1.2 GHz bandwidth amplifier, filters with corner frequencies above 100 MHz have been built. This application note will provide a recipe to realize active filters using low cost, high speed operational amplifiers. The filter topology that is used is the Sallen-Key Filter which uses the operational amplifier as a fixed gain block. This filter topology can be used with current feed-back or voltage feedback amplifiers, and you will be shown how to design low pass, high pass and band pass filters using this filter topology. This application note discusses a printed circuit board which will allow you to prototype active filters of up to 8 pole complexity (As of 1/13/05, board is no longer avail- able). This is not a complete treatment of the subject of active filters, there are several good books published on that topic, some of which are listed in the bibliography. This is a simple method of getting a circuit that will work for most applications. The topics covered are: 1) Filter Types 2) Introduction to the second order, low pass Sallen-Key Filter 3) RC:CR Transformations to realize second order, high pass Sallen-Key Filters. 4) Cascading filter sections to achieve higher order filters and Band Pass Filters 5) Sensitivities to component values 6) How to select an Op-amp for use in an active filter 7) Compensation for finite Bandwidth Amplifiers 8) The National active filter evaluation boards (As of 1/13/05, board is no longer available) Filter Types There are a large number of different types of filter re- sponses. In this application note, we will concern ourselves with the three which I have found to be the most useful. Butterworth Response The Butterworth response maximizes the flatness in the pass band of the filter. The response is very flat near DC then begins to slowly roll off so that it is at -3 dB at the cutoff frequency, and approaches an ultimate rolloff rate of -20ndB/ decade where n is the order of the filter. Butterworth filters are used it is very important to maintain gain flatness, espe- cially at low frequencies.
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ECE3042 OA-26 - Designing Active High Speed Filters...

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