AN-1698 Datasheet, PDF (1/8 Pages) National Semiconductor (TI) – A Specification for EMI Hardened Operational Amplifiers

A Specification for EMI
Hardened Operational
Amplifiers
National Semiconductor
Application Note 1698
Gerrit de Wagt and Arie van Staveren
September 2007
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 National Semiconductor developed
op amps with increased EMI robustness to overcome the is-
sues of electromagnetic interference. Along with these EMI
hardened op amps a parameter has been introduced to un-
ambiguously specify the EMI robustness of an op amp: EMI
Rejection Ratio (EMIRR). This application note presents the
background, details and usage of the EMIRR parameter.
The next section starts with a description of how RF signals
can be picked up and transferred to the op amp pins. Subse-
quently, a qualitative description of the interaction of the RF
signal and the op amp is given. To be able to compare differ-
ent op amps on their EMI robustness, the EMI Rejection Ratio
(EMIRR) is defined. The EMIRR is a parameter that quanti-
tatively 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.
EMI and Op Amps
To be able to describe the performance of op amps with re-
spect to their EMI robustness, firstly 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). Sec-
ondly, the actual interaction between the received signal at
the op amp pins and the op amp circuitry need to be consid-
ered.
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 de-
vice itself picks up the EM waves. Whether this will happen
depends on the frequency of the EM wave and the suscepti-
bility of the electronic device for that frequency. This suscep-
tibility 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, i.e. antenna for EM waves. Subse-
quently, 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 con-
ductive way at the op amp pins. These conductive distur-
bances 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 suf-
ficient to consider conductively received disturbances. So,
conductive measurements suffice to determine the EMI ro-
bustness 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 sig-
nals with a frequency that falls outside the band of the op amp
circuit, i.e. 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-con-
verted 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_V = 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.
30034901
FIGURE 1. Offset Voltage Variation Due to a Detected RF
Signal
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.
EMIRR Definition
To identify EMI robust op amps, a parameter is needed that
quantitatively describes the EMI performance. A quantitative
measure enables the comparison and the ranking of op amps
on their EMI robustness. This application note introduces the
EMI Rejection Ratio (EMIRR). This parameter describes the
resulting input-referred offset voltage shift of an op amp as a
result of an applied RF carrier (interference) with a certain
frequency and level. The definition of EMIRR is given by:
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