In most real-world control applications, we need to use sensors to transform physical variables (heat, pressure, motor shaft position, etc.) to electrical variables (voltages or currents). Only then the signals could be sampled and processed by a computer. As it normally happens, the signal output from a sensor might not be clean enough to accurately represent the quantity it measures. Very likely that it would be contaminated by high-frequency noise, or undesired disturbance with fixed frequency (interference from 50 Hz household appliance, for example). At the other analog-digital junction, the direct signal from DAC module might need to be smoothen by a low-pass filter. For a low-cost application, a passive circuit may be used for signal conditioning purpose. The drawback is lack of impedance buffering and signal amplification. If your sensor or DAC has a limited sourcing capability, an active circuit using operational amplifier (op-amps) is a better choice.
This brief technical article summarizes the use of op-amps in certain signal conditioning circuits. We only provide basic derivations, circuit examples, and simulation results, leaving detailed analysis to standard textbooks.