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Introduction

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1 Introduction

The number of electronic (mobile) devices in the world is still increasing. With this increase of transmitting

devices, the electromagnetic interference (EMI) between those devices and other equipment becomes a

bigger challenge. This raises the need for equipment and therefore integrated circuits that are more robust

to the presence of Electromagnetic waves (EM) in the air. Therefore, Texas Instruments developed op

amps with increased EMI robustness to overcome the issues of electromagnetic interference. Along with

these EMI hardened op amps a parameter has been introduced to unambiguously specify the EMI

robustness of an op amp: EMI Rejection Ratio (EMIRR).

Section 2 starts with a description of how RF signals can be picked up and transferred to the op amp pins.

Subsequently, a qualitative description of the interaction of the RF signal and the op amp is given. To be

able to compare different op amps on their EMI robustness, the EMI Rejection Ratio (EMIRR) is defined.

The EMIRR is a parameter that quantitatively describes the effect that an RF signal has on op amp

performance. The definition of EMIRR is discussed along with a straightforward method to measure the

EMIRR. Finally, two typical applications will be discussed showing the advantage of EMI hardened op

amps.

2 EMI and Op Amps

To be able to describe the performance of op amps with respect to their EMI robustness, first a model

needs to be derived that describes how the signals of disturbing (RF) sources might end up at the op amp

pins. This requires the identification of possible coupling paths from an interfering (RF) source to the op

amp (electronic victim device). Second, the actual interaction between the received signal at the op amp

pins and the op amp circuitry need to be considered.

An interfering or disturbing (RF) signal can arrive at the op amp via two different types of coupling paths:

• Radiation

• Conduction

Interference via radiation arises when an electronic victim device itself picks up the EM waves. Whether

this will happen depends on the frequency of the EM wave and the susceptibility of the electronic device

for that frequency. This susceptibility largely depends on the size of the electronic victim device relative to

the wavelength of the disturbing EM waves.

In the case of interference via conduction, other devices, such as cables and PCB traces connected to the

victim device, act as the receiving device, that is, antenna for EM waves. Subsequently, the received

signals (voltages and currents) are transferred in a conductive way to the victim device.

Since the dimensions of an op amp IC are so small (a few mm) compared to the wavelength of the

disturbing RF signals (several cm in the GHz range to tens of cm in the hundreds of MHz range),

disturbances will dominantly arrive in a conductive way at the op amp pins. These conductive

disturbances on the pin of the op amp can be represented by (RF) voltages and currents which are

received by the PCB and connecting wires. These voltages and currents might interfere with the op amp

and jeopardize proper behavior. The fact that disturbances arrive mainly in a conductive way implies that,

when determining the EMI robustness of an op amp, it is sufficient to consider conductively received

disturbances. So, conductive measurements suffice to determine the EMI robustness of op amps. No tests

need to be performed in expensive EMI chambers.

RF signals interfere with op amps via the non-linearity of the op amp circuitry. The highest non-linearity is

obtained for signals with a frequency that falls outside the band of the op amp circuit, that is, for

frequencies at which the overall feedback is virtually zero. This non-linearity results in the detection of the

so called out-of-band signals. The obtained effect is that the amplitude modulation of the out-of-band

signal is down-converted into the base band. This base band can easily overlap with the band of the op

amp circuit.

As an example, Figure 1 shows the equivalent input offset voltage of an op amp for a detected RF carrier

with on-off keying. It is assumed that the op amp is connected in unity gain (A

= 1) which means that the

obtained output voltage variation is equivalent to the input offset voltage variation. Clearly the offset

voltage varies in the rhythm of the on-off keying of the RF carrier.

The key in describing the EMI robustness of an op amp is to link the level of the applied RF signal to the

resulting level of offset voltage variation.

AN-1698 A Specification for EMI Hardened Operational Amplifiers SNOA497B–September 2007–Revised April 2013

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