More on the Non-inverting Amplifier

The circuit of Fig.1 has resistors R3 and R4. The amplifier would still work if R4 was removed and R3 is set to zero, but the amplifier will be more susceptible to what is known as common mode noise when the circuit is simplied to this extent. Common mode noise is defined as noise that appears in equal phase and amplitude on both inputs. This can happen if we have long traces, high impedance components, improper grounding and decoupling. Care should be taken to use short traces, the lowest practical values for resistors, proper grounding and capacitors connected to and placed close to the op-amp power pins to de-couple noise to ground. Resistor noise is proportional to the square-root of the resistance value. More on the subject of noise later.