LMV721M7 National Semiconductor, LMV721M7 Datasheet - Page 9

• Manufacturer Part Number: LMV721M7
• Description: IC, OP-AMP, 10MHZ, 5.25V/µs, SC-70-5
• Manufacturer: National Semiconductor

Specifications of LMV721M7

• Op Amp Type: Low Power
• No. Of Amplifiers: 1
• Bandwidth: 10MHz
• Slew Rate: 5.25V/µs
• Supply Voltage Range: 2.2V To 5.5V
• Amplifier Case Style: SC-70
• No. Of Pins: 5
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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 systems.
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 particularly
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 with-
out oscillation. The unity-gain follower is the most sensitive
configuration to capacitive loading. Direct capacitive loading
reduces the phase margin of amplifiers. The combination of
the amplifier's output impedance and the capacitive load in-
duces phase lag. This results in either an underdamped pulse
response or oscillation. To drive a heavier capacitive load,
circuit in Figure 1 can be used.
In Figure 1, the isolation resistor R
C
margin to the overall system. the desired performance de-
pends on the value of R
the more stable V
of Figure 1 using 100kΩ for R
FIGURE 1. Indirectly Driving A capacitive Load Using
L
form a pole to increase stability by adding more phase
OUT
Resistive Isolation
will be. Figure 2 is an output waveform
ISO
. The bigger the R
ISO
and 2000µF for C
ISO
and the load capacitor
ISO
10092218
resistor value,
L
.
9
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
ure 3, R
techniques to connect V
ing the value of R
LMV721/722. C
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 loop. In-
creased capacitive drive is possible by increasing the value
of C
3.0 INPUT BIAS CURRENT CANCELLATION
The LMV721/722 family has a bipolar input stage. The typical
input bias current of LMV721/722 is 260nA with 5V supply.
Thus a 100kΩ input resistor will cause 26mV of error voltage.
By balancing the resistor values at both inverting and non-
inverting inputs, the error caused by the amplifier's input bias
current will be reduced. The circuit in Figure 4 shows how to
cancel the error caused by input bias current.
FIGURE 3. Indirectly Driving A Capacitive Load with DC
FIGURE 2. Pulse Response of the LMV721 Circuit in
F
. This in turn will slow down the pulse response.
F
provides the DC accuracy by using feed-forward
F
and R
F
ISO
due to the input bias current of the
and the load resistor. Instead, in Fig-
IN
ISO
Accuracy
Figure 1
to R
serve to counteract the loss of
L
. Caution is needed in choos-
10092231
10092219
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