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Keywords: low pass filters, high pass filters, filter designs, op amps as filters, Sallen-Key filter
APPLICATION NOTE 700
Fixed-Gain Op Amps Simplify Filter Design
Jan 22, 2001
Abstract: Use less component count, money, and board space when designing Sallen-Key filters.
Simplify band pass filters with fixed gain amplfiers.
Simple second-order filters meet many filtering requirements. A low-order low-pass filter, for example, is
often adequate for antialiasing in ADC applications or for eliminating high-frequency noise in audio
applications. Similarly, a low-order high-pass filter can easily remove power-supply noise. When you
design such filters with built-in gain, fixed-gain op amps can save space, cost, and time. Figure 1
illustrates the use of fixed-gain op amps in building second-order low-pass and high-pass Sallen-Key
filters. Filter "cookbooks" are useful in designing these filters, but the cookbook procedures usually break
down for a given response, such as Butterworth, if the gain set by R
F
and R
G
is greater than unity.
What's more, the cookbook component-value formulas can yield unrealistic values for the capacitors and
the resistors.
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