The OPAx186 uses integrated electromagnetic interference (EMI) filtering to reduce the effects of EMI interference from sources such as wireless communications and densely-populated boards with a mix of analog signal chain and digital components. EMI immunity can be improved with circuit design techniques; the OPAx186 benefits from these design improvements. Texas Instruments has developed the ability to accurately measure and quantify the immunity of an operational amplifier over a broad frequency spectrum extending from 10 MHz to 6 GHz. Figure 7-2 shows the results of this testing on the OPAx186. Table 7-1 lists the EMIRR +IN values for the OPAx186 at particular frequencies commonly encountered in real-world applications. Table 7-1 lists applications that can be centered on or operated near the particular frequency shown. See also the EMI Rejection Ratio of Operational Amplifiers application report, available for download from www.ti.com.

Figure 7-2 EMIRR Testing

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 nearly matching EMIRR performance
• EMIRR is more simple to measure on noninverting pins than on other pins because the noninverting input terminal 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 can result in adverse effects, as there is insufficient amplifier loop gain to correct for signals with spectral content outside the bandwidth. Conducted or radiated EMI on inputs, power supply, or output can 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.

Figure 7-2 shows the EMIRR +IN of the OPAx186 plotted versus frequency. The OPAx186 unity-gain bandwidth is 750 kHz. EMIRR performance less than this frequency denotes interfering signals that fall within the op-amp bandwidth.