7.3.7 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 nearly matching

EMIRR performance.

• EMIRR is easier 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 result in adverse effects, as

the amplifier would not have sufficient loop gain to correct for signals with spectral content outside the amplifier

bandwidth. Conducted or radiated EMI on inputs, power supply, or output may result in unexpected dc offsets,

transient voltages, or other unknown behavior. Be sure to properly shield and isolate sensitive analog nodes

from noisy radio signals and digital clocks and interfaces. Figure 7-5 shows the effect of conducted EMI to the

power supplies on the input offset voltage of OPAx145.

The EMIRR IN+ of the OPAx145 is plotted versus frequency as shown in Figure 7-4. The OPAx145 unity-gain

bandwidth is 5.5 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.

0

20

40

60

80

100

120

140

10M

100M

1000M

Frequency (Hz)

C004

Figure 7-4. OPAx145 EMIRR IN+

±200

±150

±100

±50

0

50

100

1k

10k

100k

1M

Frequency (Hz)

C006

V± Supply

V+ Supply

Figure 7-5. OPAx145 EMI-Induced Input Offset

Voltage (Power Supplies)

www.ti.com

OPA145,, OPA2145

SBOS427E – JUNE 2017 – REVISED OCTOBER 2020

Copyright © 2020 Texas Instruments Incorporated

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Product Folder Links: OPA145, OPA2145