TABLE OF CONTENTS Steve Winder
| Note | Everybody needs a lowpass filter occasionally. You need to get some nasty high-frequency signal attenuated, and if you are going to feed it to an ADC, you might need an antialiasing filter. Depending on whether your troublesome frequency is near your primary signal or farther out, you might be able to get away with one or two capacitors or you might need a well-tuned filter with two or four or more stages. Which filter is best for you? Steve Winder has good advice. Fortunately, it only takes a few op-amps and a few passive components to confirm how much filtering you need. Furthermore, there are free computer-based programs to help you choose and design your filter. /rap |
This chapter describes how to design active or passive low pass filters to almost any desired specification. Formulae and examples of how to use them are given for the denormalization of component values.
You will sometimes need a filter that will attenuate some unwanted signals while it passes your desired signal straight through. Filters can be made of inductors and capacitors ("passive" filters made of L and C) or resistors and capacitors, usually with an "active" amplifier such as a transistor or op-amp. Filtering can also be done with digital filters (digital signal processing, or DSP) or with switched capacitors, which we will not discuss, since those are specialties.
There are lowpass filters to reject high-frequency noise and
