OPA186DBVT datasheet(20/48 Pages) TI1

20 / 48 page

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.

7.3.4.1 EMIRR +IN Test Configuration

Figure 7-3 shows the circuit configuration for testing the EMIRR +IN. An RF source is connected to the op-amp

noninverting input pin using a transmission line. The op amp is configured in a unity-gain buffer topology with

the output connected to a low-pass filter (LPF) and a digital multimeter (DMM). A large impedance mismatch

at the op amp input causes a voltage reflection; however, this effect is characterized and accounted for when

determining the EMIRR IN+. The multimeter samples and measures the resulting dc offset voltage. The LPF

isolates the multimeter from residual RF signals that can interfere with multimeter accuracy.

Digital

Multimeter

Ambient temperature: 25Û&

Sample /

Averaging

V+

V±

Not shown: 0.1 µF and 10 µF supply decoupling

RF Source

DC Bias: 0 V

Modulation: None (CW)

Frequency Sweep: 201 pt. Log

Low-Pass

Filter

Figure 7-3. EMIRR +IN Test Configuration

OPA186, OPA2186, OPA4186

SBOS968C – JUNE 2022 – REVISED JULY 2023

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