INA828: EMI Rejection

Ohno-san,

Radiated noise emissions are generated when electrons accelerate and deaccelerate. It propagates predominantly by the electromagnetic field associated with the changing current. Therefore when current is changing inside a circuit, including inside an integrated circuit, the potential for emissions exists. But even thought the mechanism is in place for this to occur inside integrated circuits, producing fields of an observable intensity is difficult. Not only must the fields be of observable intensity, but they must be able to propagate effectively to other points in a circuit where they become an interference issue.

Emissions from integrated circuits is most often observed where very fast switching is occurring and the current levels are high. Fast edge rates on the switching waveforms increases the frequency content of the emissions, and high current increases the field intensities. I have seen examples on line where radiated emissions are being detected on mega-gate, very fast, high current processors. In such cases it is much easier to detect the emitted fields than it is with a low power gate that is switching.

Linear amplifier integrated circuits such as the INA828 do not involve any internal switching. All nodes internal to the INA828 function in a linear manner. We do not expect any fields of a perceptible level to be generated by it.

What we do measure on many of our Precision Amplifier products is the Electromagnetic Interference Rejection Ratio - EMIRR. This is a good measure of the amplifier's susceptibility to, or rejection of conducted EMI injected into its inputs. Most often this is the EMI concern when precision amplifiers are operating in close proximity to other circuit elements that produce RF fields such as that created by processors, digital and switching circuits, oscillators and RF generating circuits. Section 7.3.2 in the INA828 datasheet, "EMI Rejection," provides thorough information about the amplifier's EMIRR performance across frequency.

Regards, Thomas

Precision Amplifiers Applications Engineering