Filter

The most basic filter circuits are the CR high pass and RC low Pass filters. The high pass filter removes DC and low frequencies and the low pass removes high frequencies.

The way to remember the two circuits is to think about how the capacitor acts at various signal frequencies. At high frequencies capacitors act like wires. At low frequencies capacitors act like an open circuit (disconnected).

An important use of low pass filters is for the anti-aliasing circuity on the input of an A/D converter or the output of a D/A converter. An important use of a high pass filter is to remove the DC offset of a signal. For example, it's possible to use a power supply that is +5 Volts to GND to generate a sine wave from a D/A converter and use a high pass filter to change the sine wave to a +2.5 to -2.5 Volt signal at the output.