8.1.4 EMI Rejection

The electromagnetic interference (EMI) rejection ratio, or EMIRR, describes the EMI immunity of operational amplifiers. An adverse effect that is common to many op amps is a change in the offset voltage as a result of RF signal rectification. An op amp that is more efficient at rejecting this change in offset as a result of EMI has a higher EMIRR and is quantified by a decibel value. Measuring EMIRR can be performed in many ways, but this section provides the EMIRR IN+, which specifically describes the EMIRR performance when the RF signal is applied to the noninverting input pin of the op amp. In general, only the noninverting input is tested for EMIRR for the following three reasons:

• Op amp input pins are known to be the most sensitive to EMI, and typically rectify RF signals better than the supply or output pins.
• The noninverting and inverting op amp inputs have symmetrical physical layouts and exhibit approximately matching EMIRR performance
• EMIRR is easier to measure on noninverting pins than on other pins because the noninverting input pin can be isolated on a PCB. This isolation allows the RF signal to be applied directly to the noninverting input terminal with no complex interactions from other components or connecting PCB traces.

High-frequency signals conducted or radiated to any pin of the operational amplifier result in adverse effects, as the amplifier does not have sufficient loop gain to correct for signals with spectral content outside its bandwidth. Conducted or radiated EMI on inputs, power supply, or output may result in unexpected DC offsets, transient voltages, or other unknown behavior. Take care to properly shield and isolate sensitive analog nodes from noisy radio signals and digital clocks and interfaces.

The EMIRR IN+ of the OPA169x amplifiers is plotted versus frequency as shown in Figure 58. If available, any dual and quad op amp device versions have nearly similar EMIRR IN+ performance. The OPA169x unity-gain bandwidth is 5.1 MHz. EMIRR performance below this frequency denotes interfering signals that fall within the op amp bandwidth.

See EMI Rejection Ratio of Operational Amplifiers, available for download from www.ti.com.

Table 1 lists the EMIRR IN+ values for the OPA169x at particular frequencies commonly encountered in real-world applications. Applications listed in Table 1 may be centered on or operated near the particular frequency shown. This information may be of special interest to designers working with these types of applications, or working in other fields likely to encounter RF interference from broad sources, such as the industrial, scientific, and medical (ISM) radio band.