FAQ-OPAMP-0014

Q:

I am trying to configure an audio circuit using an op. amp. How can I minimise the noise?

A:

First of all, you will need to lower the source resistance to avoid the thermal noise. This does not mean just lowering the output resistance of pre - stage but reducing all resistance values that are connected to the input terminal. Therefore, it is also required to lower resistance in the feedback circuit. Also, by cutting down impedance of each part of the circuit, interference also can be minimised. 1kΩ resistance generates 4nV/√Hz thermal noise at room temperature, so the NJM5532-class op. amp could lose half of its Proficiency in low noise characteristics if the resistance is connected in sevies at input Generally, a low-noise, bipolar transistor input op. amp features a large bias current and generates current noise that is proportional to the square root of the base current of the input transistor.

The product of the eguivalent input noise current multiplied by the source resistance will be added to the eguivalent input noise voltage. If you cannot cut down source resistance, it may be more useful to use a J-FET input op. amp featuring a small input bias current such as NJM072B and NJM2082, than to use a bipolar transistor input op. amp featuring a smaller eguivalent input noise voltage.

Basically, for package mounting grounding is one-point earth, and please check that the current pass and signal pass for noise sources do not have a common impedance. Furthermore, there are a lot of guidelines for package mounting, such as "For making supply circuit low noise" etc, but for details please refer to specialised books.