Lecture 42

Lecture 42 - Lecture 42 Active filters Example filter...

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Lecture 42 Active filters
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Example filter design • Have several specs: • In-band gain (often>>1) • Out-of band rejection • In-band flatness • Example: LPF – Adc=60 (36dB) – BW=100kHz – Less than 25% in-band gain variation (2dB) – Need to suppress signals at f>400kHz by 36dB (relative to wanted) 36dB -36dB 100kHz 400kHz Need to drop by 36dB from 100khz to 400kHz Just as you get -20dB/decade per pole. Also get -6dB/octave per pole (octave is x2 change in frequency)
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Example filter design • To meet suppression spec over 2 octaves: get - 12dB/pole, – need 3 poles at 100kHz • If all three real: get -9dB at 100kHz: fails flatness • So need complex conjugate poles as well as real – Choose Q~1.4 to balance roll-off from real pole – This works… but how implement? – And need gain -36dB 100kHz 400kHz Out-of band rejection In band flatness ( 29 60 , 4 . 1 , 100 2 2 2 2 = = = + + + = G Q kHz Q s s s G s H o o o o o o π ϖ
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Lecture 42 - Lecture 42 Active filters Example filter...

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