Filters are useful when processing data signals. Low pass often used to eliminate noise, high pass filters eliminate static signals and leave dynamic signals. Band pass filters reject all frequencies outside a desired frequency band. A low pass filter is shown in Figure 7.37 Low-Pass Filter. At high frequencies the capacitor, C, has a very low impedance, and grounds the input signal. At low frequencies the capacitor impedance is high, increasing the gain of the op-amp circuit. This is easier to conceptualize if the R1-C pair are viewed as a voltage divider.
A high pass filter is shown in Figure 7.38 High-Pass Filter. In this case the voltage divider in the previous circuit is reversed. In this circuit the gain will increase for signals with higher frequencies.