LMV715MF NSC [National Semiconductor], LMV715MF Datasheet - Page 12

• Manufacturer Part Number: LMV715MF
• Description: Low Power, RRIO Operational Amplifiers with High Output Current Drive and Shutdown Option
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Note
4.0 COMPENSATION OF INPUT CAPACITANCE
In the application (Figure 4) where a large feedback resistor
is used, the feedback resistor can react with the input ca-
pacitance of the op amp and introduce an additional pole to
the close loop frequency response.
This pole occurs at frequency f
Any stray capacitance due to external circuit board layout,
any source capacitance from transducer or photodiode con-
nected to the summing node will also be added to the input
capacitance. If f
bandwidth (5MHz) of the op amp, the phase margin of the
loop is reduced and can cause the system to be unstable.
To avoid this problem, make sure that f
octaves beyond the expected −3dB frequency corner of the
close loop frequency response. If not, a feedback capacitor
C
The paralleled R
the effect from the pole.
5.0 CAPACITIVE LOAD TOLERANCE
The LMV710, LMV711 and LMV715 can directly drive 200pF
in unity-gain without 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 induces phase lag. This results in either an
underdamped pulse response or oscillation. To drive a
heavier capacitive load, circuit in Figure 5 can be used.
FIGURE 4. Cancelling the Effect of Input Capacitance
F
can be placed in parallel with R
p
F
and C
is less than or close to the unity-gain
F
introduce a zero, which cancels
(Continued)
p
, where
F
such that
p
10132518
occurs at least 2
12
In Figure 5, the isolation resistor R
C
margin to the overall system. The desired performance de-
pends on the value of R
value, the more stable V
not great when the R
resistor in Figure 5, the output would be voltage divided by
R
The circuit in Figure 6 is an improvement to the one in Figure
5 because it provides DC accuracy as well as AC stability. In
this circuit, R
forward techniques to connect V
to counteract the loss of phase margin by feeding the high
frequency component of the output signal back to the ampli-
fier’s inverting input, thereby preserving phase margin in the
overall feedback loop. Increased capacitive drive is possible
by increasing the value of C
pulse response.
6.0 APPLICATION CIRCUITS
PEAK DETECTOR
Peak detectors are used in many applications, such as test
equipment, measurement instrumentation, ultrasonic alarm
systems, etc. Figure 7 shows the schematic diagram of a
peak detector using LMV710 or LMV711 or LMV715. This
peak detector basically consists of a clipper, a parallel RC
network, and a voltage follower.
FIGURE 6. Indirectly Driving A Capacitive A Load with
FIGURE 5. Indirectly Driving A Capacitive Load using
L
ISO
form a pole to increase stability by adding more phase
and the load resistor.
F
provides the DC accuracy by using feed-
Resistive Isolation
ISO
DC Accuracy
OUT
ISO
gets bigger. If there were a load
F
. The bigger the R
will be. But the DC accuracy is
. This in turn will slow down the
IN
ISO
to R
and the load capacitor
L
. C
10132521
F
and R
10132522
ISO
ISO
resistor
serve