Datasheet

¸
¸
¹
·
¨
¨
©
§
'V
OS
V
RF_PEAK
EMIRR
V
RF_PEAK
= 20 log
RF SIGNAL
V
OUT OPAMP
(A
V
= 1)
NO RF
RF
V
OS
+ V
DETECTED
V
OS
LMV851, LMV852, LMV854
SNOSAW1A –OCTOBER 2007–REVISED MARCH 2013 www.ti.com
EMIRR
With the increase of RF transmitting devices in the world, the electromagnetic interference (EMI) between those
devices and other equipment becomes a bigger challenge. The LMV851/LMV852/LMV854 are EMI hardened op
amps which are specifically designed to overcome electromagnetic interference. Along with EMI hardened op
amps, the EMIRR parameter is introduced to unambiguously specify the EMI performance of an op amp. This
section presents an overview of EMIRR. A detailed description on this specification for EMI hardened op amps
can be found in Application Note AN-1698(SNOA497).
The dimensions of an op amp IC are relatively small compared to the wavelength of the disturbing RF signals. As
a result the op amp itself will hardly receive any disturbances. The RF signals interfering with the op amp are
dominantly received by the PCB and wiring connected to the op amp. As a result the RF signals on the pins of
the op amp can be represented by voltages and currents. This representation significantly simplifies the
unambiguous measurement and specification of the EMI performance of an op amp.
RF signals interfere with op amps via the non-linearity of the op amp circuitry. 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 52 depicts a typical output signal of a unity-gain connected op amp in the
presence of an interfering RF signal. Clearly the output voltage varies in the rhythm of the on-off keying of the RF
carrier.
Figure 52. Offset Voltage Variation Due to an Interfering RF Signal
EMIRR Definition
To identify EMI hardened op amps, a parameter is needed that quantitatively describes the EMI performance of
op amps. A quantitative measure enables the comparison and the ranking of op amps on their EMI robustness.
Therefore the EMI Rejection Ratio (EMIRR) is introduced. 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:
(1)
In which V
RF_PEAK
is the amplitude of the applied un-modulated RF signal (V) and ΔV
OS
is the resulting input-
referred offset voltage shift (V). The offset voltage depends quadratically on the applied RF level, and therefore,
the RF level at which the EMIRR is determined should be specified. The standard level for the RF signal is 100
mV
P
. Application Note AN-1698(SNOA497) addresses the conversion of an EMIRR measured for an other signal
level than 100 mV
P
. The interpretation of the EMIRR parameter is straightforward. When two op amps have an
EMIRR which differ by 20 dB, the resulting error signals when used in identical configurations, differs by 20 dB
as well. So, the higher the EMIRR, the more robust the op amp.
Coupling an RF Signal to the IN
+
Pin
Each of the op amp pins can be tested separately on EMIRR. In this section the measurements on the IN
+
pin
(which, based on symmetry considerations, also apply to the IN− pin) are discussed. In Application Note AN-
1698(SNOA497) the other pins of the op amp are treated as well. For testing the IN
+
pin the op amp is
connected in the unity gain configuration. Applying the RF signal is straightforward as it can be connected
directly to the IN
+
pin. As a result the RF signal path has a minimum of components that might affect the RF
signal level at the pin. The circuit diagram is shown in Figure 53. The PCB trace from RF
IN
to the IN
+
pin should
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