Part 1 of this series addressed a pitfall to be avoided when using Sallen-Key active filters. Please also see my article Avoid Misuse of Sallen-Key Filters.

The key point of that essay was that the op-amp of the Sallen-Key circuit could not respond in zero time to stepped signal inputs having zero rise and fall times. In point of fact, any signal input with sufficiently fast dV/dT properties could/will cause a Sallen-Key lowpass filter to malfunction. The same point is also to be made for the zero offset lowpass filter previously described in part 1.

The suggested remedy for the Sallen-Key lowpass filter was/is the use of a passive RC at the signal input to reduce the speed of response demand placed on the op-amp. That is also the suggested remedy for the zero offset lowpass filter.

A SPICE simulation will serve to demonstrate.

Figure 1 Two zero offset lowpass filters

In the above figure, two filter are shown; the upper one, based on the prior essay, and the lower one with the addition of a passive RC at the signal input. The frequency responses of the two circuits are compared as follows.

Figure 2 Frequency response comparison

Although the upper circuit has two poles while the lower circuit has three poles, the frequency response curves of the two circuits are virtually indistinguishable from each other near the dominant corner frequency, but the signal output responses of the two circuits, to square wave signal inputs, are markedly different from each other.

Figure 3 Signal output comparison

As in the Sallen-Key case, the passive RC limits the speed of response demand placed on the op-amp so that the unwanted output glitches are not generated.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).