The circuit for a second order filter and its transfer function with 2 poles and 2 zeros is shown below. C7-R7 can be added to the output of the circuit to produce a third order section. With R1 = R4, the zeros are on the imaginary axis and by choice of components, the zeros can be at lower frequencies than the poles. This makes the circuit suitabe for a high pass filter. This circuit is expensive in opamps but a current diffrerence opamp can be used to replace the integrator and inverter, reducing the number of opamps by 1.

Image below shows component values for a 7th order Elliptic low pass filter implemented as 2 second order and 1 third order section.

r = 25 and q = 29°.

R is set to 100KW, C = 0.01 mF and the 3dB frequency is 3kHz.

The normalized transition BW is ~0.516rad/s, the minimum stop band attenuation is > 100dB and the pass band ripple is 0.28dB.

COPYRIGHT © 2011 Cuthbert Nyack.