ABSTRACT
This chapter examines two important aspects of performance; the noise and distortion behaviour of various types of filter. Distortion in active filters may come from the amplifiers or the capacitors. The basic amplifier distortion is will be increased in a Sallen & Key filter by common-mode distortion, due to significant series resistance in lowpass case, and in both lowpass and highpass cases by the fact that the amplifier inputs see the full signal amplitude. The chapter turns from the effect of amplifiers to effect of capacitor non-linearity on active filter distortion performance. In active filters distortion may come from the amplifiers and the capacitors. Filters of order three and higher are commonly built using 2nd-order and 1st-order stages as building blocks, and the noise output is simply the rms-sum of the cascaded stages. Filters that implement 3rd- and 4th-order responses in a single stage have rather different noise behaviours, in the same way that they have rather different distortion characteristics.
