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ac-dc,
Sallen-Key is simply a hardware implementation of a low-pass or hgh pass filter. Low pass filters are used in applications where low frequencies must be passed, and high frequencies must be attenuated. Conversely, high pass filters are used in applications where high frequencies must be passed, but low frequencies must be attenuatied.
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The choice of a Sallen-Key versus other implementations of the low-pass or high-pass function depends on many factors. For example, if sensitivity to component variations is important, the state variable implementation might be a better choice, even though it requires 3 or 4 op-amps per section vs 2 for the Sallen Key. In very high frequency applications, you must use a passive L-C filter because no op- amps are available with the reuired bandwidth.
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Google on "Active Filters" for more details.
Regards,
Kral
ac-dc,
I can give you three examples of my own designs:
A demodulator is a circuit that converts AC into half wave or full wave rectified DC. I used a 2nd order Butterworth filter implemented in the Sallen-Key configuration to convert the rectified AC into ripple-free DC.
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An active crossover is a filter used in high fidelity audio systems to separate low frequencies and high frequencies. The low frequencies go the the woofer. The high frequencies go to the tweeter. The active filter consists of a low pass filter which provides the output to the woofer (after being amplified by a power amplifier), and a high pass filter which provides output to the tweeter (again after being amplified by a power amplifier. Both the low-pass and high pass filters are implemented using the Sallen-Key hardware configuration. Both filters are 4-pole filters that are implemented using 2 sections, each section implemented using the Sallen-Key hardware configuration. The first section provides one set of poles, the 2nd section provides the 2nd pair of poles. each section is a Butterworth filter. This configuration of 2 cascaded Butterworth filters is know as a Linkwitz-Riley, crossover, after its inventors.
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The third application uses a Sallen-Key filter to filter high frequency noise from a DC signal before it is applied to the input of an A to D Converter.
Regards,
Kral
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