3.3 Low-Pass RC Filter
The purpose of this circuit is to attenuate high frequency
fluctuations in voltage while leaving those at low frequency
largely unaffected. This circuit is easy to understand if
we assume it is connected to a device with a high input impedance,
i.e. if we assume that iout = 0. In this case
the difference between the input and output voltages is the voltage
across the resistor, i.e.
vin - vout = iin R
where iin is also the current charging the capacitor.
If vin is constant then the capacitor will soon become charged,
iin will become zero and we will have vout = vin.
If vin fluctuates slowly then iin will be small
(capacitor only charging slowly) and we will still have, approximately, vout = vin. As the frequency of fluctuations increases
though, and the rate of change of vin becomes comparable to the speed
at which the capacitor can charge, iin will become significant and
thus vout < vin. This circuit therefore has a dynamic
response that is a function of frequency.