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To talk about filters, we need to remind ourselves about the time domain and the frequency domain.   The time domain simply describes how a voltage level changes over time.  The frequency domain describes the different frequencies or oscillations that are present in the signal.  Voltage signals can be divided into two broad categories.  The first is DC or direct current.  It has a fixed level over time.  All other signals have a frequency component associated with them, we call them time varying; not to be confused with AC or alternating current which is a specific group of time varying signals that have a sinusoidal shape.
The notion of filtering comes from the idea that certain parts of a signal will be allowed to pass the filter while other parts will be blocked.  In the frequency domain, where zero frequency indicates a DC signal, a low pass filter allows all frequencies from DC up to a certain point, called the cutoff frequency, to pass and blocks, or attenuates, signals that have higher frequencies.  A high pass filter is just the opposite.  It allows high frequency signals to pass but blocks or attenuates low frequencies.  A band pass filter allows frequencies of a certain range to pass.  It will have upper and lower cutoff frequencies that define the pass band.  A notch filter is the opposite of a bandpass filter.  It has a region or band of frequencies that it blocks.  Again it will have upper and lower cutoff frequencies that define the notch.
Filters can be active or passive.  Passive filtering uses resistors, capacitors and inductors to create the filter.  An active filter will have active components such as an amplifier.  The amplifier can be made of devices such as op amps or transistors.  It can be a simple amplifier or a very complex one.
An important concept in filter design is the idea of filter order.  The filter order is simply the number of poles in the filter.  A first order filter will have one pole. An RC filter, made of single resistor and a single capacitor, is an example of a first order filter.  An ideal filter would pass frequencies up to a certain cutoff frequency and then block frequencies past that cutoff frequency.  In reality the sharpness of the filter is called its roll off and is related to the filter order.  The higher the order of the filter the faster, or sharper, the transition from passing to blocking will be.
One type of filter that is commonly used in mixed signal designs is the anti-aliasing filter.  It is used when a signal is to be converted from analog to digital by, for example, an A to D converter on a microcontroller.  In fact, it is absolutely essential that one be used when digitizing a signal.  When the signal is converted it is sampled at a specific frequency, e.g. 100 Hz.  If the incoming signal has frequencies higher than 50 Hz the digitized signal will be aliased, that is it will have distortion and errors that mean it is not an accurate representation of the original signal.
In the future there will be more articles on filtering, types of filters, filter flatness, phase delay and more.

Final thoughts
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