LMV852EVAL National Semiconductor, LMV852EVAL Datasheet - Page 15

• Manufacturer Part Number: LMV852EVAL
• Description: BOARD EVALUATION LMV852 8MHZ
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LMV852EVAL

• Channels Per Ic: 2 - Dual
• Amplifier Type: General Purpose
• Output Type: Single-Ended, Rail-to-Rail
• Slew Rate: 4.5 V/µs
• Current - Output / Channel: 65mA
• Operating Temperature: -40°C ~ 125°C
• Current - Supply (main Ic): 820µA
• Voltage - Supply, Single/dual (±): 2.7 V ~ 5.5 V, ±1.35 V ~ 2.75 V
• Board Type: Fully Populated
• Utilized Ic / Part: LMV852
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• -3db Bandwidth: -

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 7 mV from the positive and negative rail
The LMV851/LMV852/LMV854 can be connected as non-in-
verting unity gain amplifiers. This configuration is the most
sensitive to capacitive loading. The combination of a capaci-
tive load placed at the output of an amplifier along with the
amplifier’s output impedance creates a phase lag, which re-
duces the phase margin of the amplifier. If the phase margin
is significantly 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
LMV851/LMV852/LMV854 can directly drive capacitive loads
up to 200 pF without any stability issues. In order to drive
heavier capacitive loads, an isolation resistor, R
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
loop. The larger the value of R
fier will be. If the value of R
back loop will be stable, independent of the value of C
However, larger values of R
and reduced output current drive.
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 inter-
ference. Along with EMI hardened op amps, the EMIRR pa-
rameter 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.
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.
FIGURE 3. Isolating Capacitive Load
ISO
ISO
L
ISO
result in reduced output swing
is sufficiently large, the feed-
is no longer in the feedback
, the more stable the ampli-
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ISO
, should be
L
.
15
Clearly the output voltage varies in the rhythm of the on-off
keying of the RF carrier.
FIGURE 4. Offset Voltage Variation Due to an Interfering
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
lated RF signal (V) and ΔV
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
AN-1698 addresses the conversion of an EMIRR measured
for an other signal level than 100 mV
the EMIRR parameter is straightforward. When two op amps
have an EMIRR which differ by 20 dB, the resulting error sig-
nals 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
Each of the op amp pins can be tested separately on EMIRR.
In this section the measurements on the IN
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
is connected in the unity gain configuration. Applying the RF
signal is straightforward as it can be connected directly to the
IN
ponents that might affect the RF signal level at the pin. The
circuit diagram is shown in Figure 5. The PCB trace from
RF
the RF impedance of the cabling and the RF generator. On
the PCB a 50Ω termination is used. This 50Ω resistor is also
used to set the bias level of the IN+ pin to ground level. For
determining the EMIRR, two measurements are needed: one
is measuring the DC output level when the RF signal is off;
and the other is measuring the DC output level when the RF
signal is switched on. The difference of the two DC levels is
the output voltage shift as a result of the RF signal. As the op
amp is in the unity gain configuration, the input referred offset
+
IN
pin. As a result the RF signal path has a minimum of com-
to the IN
RF_PEAK
+
pin should be a 50Ω stripline in order to match
is the amplitude of the applied un-modu-
RF Signal
OS
is the resulting input-referred
+
Pin
P
. The interpretation of
P
+
. Application Note
pin (which, based
+
pin the op amp
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