LMV861 Datasheet, PDF (14/20 Pages) National Semiconductor (TI) – 30 MHz Low Power CMOS, EMI Hardened Operational Amplifiers

OUTPUT CHARACTERISTICS
As already mentioned the output is rail-to-rail. When loading
the output with a 10 kΩ resistor the maximum swing of the
output is typically 3 mV from the positive and negative rail.
The output of the LMV861 and LMV862 can drive currents up
to 70 mA at 3.3V, and even up to 150 mA at 5V.
The LMV861 and LMV862 can be connected as non-inverting
unity gain amplifiers. This configuration is the most sensitive
to capacitive loading. The combination of a capacitive load
placed at the output of an amplifier along with the amplifier’s
output impedance creates a phase lag, which reduces the
phase margin of the amplifier. If the phase margin is signifi-
cantly reduced, the response will be under damped which
causes peaking in the transfer and, when there is too much
peaking, the op amp might start oscillating. The LMV861 and
LMV862 can directly drive capacitive loads up to 200 pF with-
out any stability issues. In order to drive heavier capacitive
loads, an isolation resistor, R_ISO, should be used, as shown
in Figure 3. By using this isolation resistor, the capacitive load
is isolated from the amplifier’s output, and hence, the pole
caused by C_L is no longer in the feedback loop. The larger the
value of R_ISO, the more stable the amplifier will be. If the value
of R_ISO is sufficiently large, the feedback loop will be stable,
independent of the value of C_L. However, larger values of
R_ISO result in reduced output swing and reduced output cur-
rent drive.
30024063
FIGURE 3. Isolating Capacitive Load
A resistor value of around 50Ω would be sufficient. As an ex-
ample some values are given in the following table, for 5V and
an open loop gain of 111 dB.
C_LOAD
R_ISO
300 pF
62Ω
400 pF
55Ω
500 pF
50Ω
When increasing the closed-loop gain the capacitive load can
be increased even further. With a closed loop gain of 2 and a
27Ω isolation resistor, the load can be 1 nF
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
LMV861 and LMV862 are EMI hardened op amps which are
specifically designed to overcome electromagnetic interfer-
ence. Along with EMI hardened op amps, the EMIRR param-
eter is introduced to unambiguously specify the EMI perfor-
mance 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.
The dimensions of an op amp IC are relatively small com-
pared 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 rep-
resented by voltages and currents. This representation sig-
nificantly 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 4 depicts a typical output signal of a unity-gain con-
nected 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.
30024065
FIGURE 4. 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 param-
eter 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:
In which V_{RF_PEAK} is the amplitude of the applied un-modu-
lated RF signal (V) and ΔV_OS is the resulting input-referred
offset voltage shift (V). The offset voltage depends quadrati-
cally 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 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 EMIRRs 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 the other pins of
the op amp are treated as well. For testing the IN+ pin the op
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