Typical Performance characteristics
Gain and Phase Margin vs.
Frequency (V
S
= 5V, R
L
600Ω)
(Continued)
Slew Rate vs.
Supply Voltage
THD vs.
Frequency
DS100922-16
DS100922-17
DS100922-42
Application Notes
1.0 Benefits of the LMV721/722 Size.
The small footprints of the LMV721/722 packages save
space on printed circuit boards, and enable the design of
smaller electronic products, such as cellular phones, pagers,
or other portable systems. The low profile of the
LMV721/722 make them possible to use in PCMCIA type III
cards.
Signal Integrity.
Signals can pick up noise between the sig-
nal source and the amplifier. By using a physically smaller
amplifier package, the LMV721/722 can be placed closer to
the signal source, reducing noise pickup and increasing sig-
nal integrity.
Simplified Board Layout.
These products help you to avoid
using long pc traces in your pc board layout. This means that
no additional components, such as capacitors and resistors,
are needed to filter out the unwanted signals due to the inter-
ference between the long pc traces.
Low Supply Current.
These devices will help you to maxi-
mize battery life. They are ideal for battery powered sys-
tems.
Low Supply Voltage.
National provides guaranteed perfor-
mance at 2.2V and 5V. These guarantees ensure operation
throughout the battery lifetime.
Rail-to-Rail Output.
Rail-to-rail output swing provides maxi-
mum possible dynamic range at the output. This is particu-
larly important when operating on low supply voltages.
Input Includes Ground.
Allows direct sensing near GND in
single supply operation.
Protection should be provided to prevent the input voltages
from going negative more than −0.3V (at 25˚C). An input
clamp diode with a resistor to the IC input terminal can be
used.
2.0 Capacitive Load Tolerance
The LMV721/722 can directly drive 4700pF in unity-gain
without oscillation. The unity-gain follower is the most sensi-
tive configuration to capacitive loading. Direct capacitive
loading reduces the phase margin of amplifiers. The combi-
nation of the amplifier’s output impedance and the capacitive
load induces phase lag. This results in either an under-
damped pulse response or oscillation. To drive a heavier ca-
pacitive load, circuit in
Figure 1
can be used.
DS100922-18
FIGURE 1. Indirectly Driving A capacitive Load Using
Resistive Isolation
In
Figure 1,
the isolation resistor R
ISO
and the load capacitor
C
L
form a pole to increase stability by adding more phase
margin to the overall system. the desired performance de-
pends on the value of R
ISO
. The bigger the R
ISO
resistor
value, the more stable V
OUT
will be.
Figure 2
is an output
waveform of
Figure 1
using 100kΩ for R
ISO
and 2000µF for
C
L
.
DS100922-31
FIGURE 2. Pulse Response of the LMV721 Circuit in
Figure 1
The circuit in
Figure 3
is an improvement to the one in
Figure
1
because it provides DC accuracy as well as AC stability. If
there were a load resistor in
Figure 1,
the output would be
voltage divided by R
ISO
and the load resistor. Instead, in
Fig-
ure 3,
R
F
provides the DC accuracy by using feed-forward
techniques to connect V
IN
to R
L
. Caution is needed in choos-
ing the value of R
F
due to the input bias current of the
LMV721/722. C
F
and R
ISO
serve to counteract the loss of
phase margin by feeding the high frequency component of
the output signal back to the amplifier’s inverting input,
thereby preserving phase margin in the overall feedback
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