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Frequency (Hz)

C004

22

OPA202

SBOS812A – OCTOBER 2017 – REVISED SEPTEMBER 2018

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7.3.9 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 is 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 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 pin

with no complex interactions from other components or connecting PCB traces.

High-frequency signals conducted or radiated to any pin of the operational amplifier may result in adverse

effects, as the amplifier does not have sufficient loop gain to correct for signals with spectral content outside

the 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. shows the effect of conducted EMI to

the power supplies on the input offset voltage of OPA202.

The EMIRR IN+ of the OPA202 is plotted versus frequency as shown in Figure 45. If available, any dual and

quad op amp device versions have similar EMIRR IN+ performance. The OPA202 unity-gain bandwidth is 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.

Figure 45. OPA202 EMIRR IN+