This is a low-pass filter implemented using a resistor and a capacitor. A low-pass filter passes lower frequencies and attenuates higher frequencies. The capacitor passes higher frequencies, causing the voltage across it to be reduced and keeping the output voltage closer to ground. The capacitor blocks lower frequencies, causing reduced current across the resistor and keeping the output voltage closer to the input voltage.
Below the circuit is the frequency response in dB for a range of frequencies. You can click on the frequency response graph to see the circuit in operation at that particular frequency.
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Analog Filter Applet