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4th order single chip filter

High-order filters are normally designed by using two or more 2nd-order sections in series. That means that a 4th-order filter needs at least two opamps. The present filter, however, uses only one opamp, which results in smaller distortion, intermodulation, and so on. Also, there is no internal resonance rise, which typifies combinations of 2nd-order sections. Because of this, the peak input signal may be equal to the peak output of the opamp (but consider the common-mode input range). Drawbacks of the circuit are the rather high ratio C3/C4 and the minimum value of the resistors. The resistor values are determined by the load on the output of the opamp (of which the resistors form a part). The maximum load (with large signals) of a TL081 is 2 k(2. Resistors RI-R.4 constitute an impedance of 2.5 162 so that the external load must not be smaller than 10 ka If an opamp is used that can handle a load of 600 C2, it is advisable to give R1-R4 a minimum value of 2.5 kQ. This will lower he noise emanating from the filter, which s generated chiefly by the resistors. le characteristic of the filter is a 4th-order 3essel polynomial. A Butterworth character-!ristic is difficult to obtain with this type I filter, since, owing to the unity amplifi-ltion of the opamp, the ratio G3:C4 .comes very high. With component values shown, the -3 dB point is 1 kHz. Cher cut-off frequencies can be obtained by recalculating the component values: these are directly proportional to the frequency. The 4th-order Bessel and Butter-worth polynomials and the transfer function of the circuit are given below.

single chip filter



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