LMV922 NSC [National Semiconductor], LMV922 Datasheet - Page 15

• Manufacturer Part Number: LMV922
• Description: 1.8V, 1MHz, Low Power Operational Amplifiers with Rail-To-Rail Input and Output
• Manufacturer: NSC [National Semiconductor]

Application Note
1.0 Unity Gain Pulse Response Considerations
The unity-gain follower is the most sensitive configuration to
capacitive loading. The LMV921/LMV922/LMV924 family
can directly drive 1nF in a unity-gain with minimal ringing. Di-
rect capacitive loading reduces the phase margin of the am-
plifier. The combination of the amplifier’s output impedance
and the capacitive load induces phase lag. This results in ei-
ther an underdamped pulse response or oscillation. The
pulse response can be improved by adding a pull up resistor
as shown in Figure 1
Higher capacitances can be driven by decreasing the value
of the pull-up resistor, but its value shouldn’t be reduced be-
yond the sinking capability of the part. An alternate approach
is to use an isolation resistor as illustrated in Figure 2 .
2.0 Input Bias Current Consideration
The LMV921/LMV922/LMV924 family has a bipolar input
stage. The typical input bias current (I
bias current can develop a significant offset voltage. This off-
set is primarily due to I
back resistor, R
R
Figure 3 , cancels this affect. But the input offset current (I
will still contribute to an offset voltage in the same manner.
= I
FIGURE 2. Using an Isolation Resistor to Drive Heavy
FIGURE 1. Using a Pull-Up Resistor at the Output for
F
B
is 100k , then an offset voltage of 5mV will develop (V
X R
F
). Using a compensation resistor (R
Stabilizing Capacitive Loads
F
. For example, if I
Capacitive Loads
B
flowing through the negative feed-
B
is 50nA (max room) and
B
) is 12nA. The input
DS100979-41
C
DS100979-43
), as shown in
OS
OS
)
15
3.0 Operating Supply Voltage
The LMV921/LMV922/LMV924 family is guaranteed to oper-
ate from 1.8V to 5.0V. They will begin to function at power
voltages as low as 1.2V at room temperature when un-
loaded. Start up voltage increases to 1.5V when the amplifier
is fully loaded (600
voltage is not guaranteed to follow the input. Figure 4 below
shows the output voltage vs. supply voltage with the
LMV921/LMV922/LMV924 configured as a voltage follower
at room temperature.
4.0 Input and Output Stage
The rail-to-rail input stage of this family provides more flex-
ibility for the designer. The LMV921/LMV922/LMV924 use a
complimentary PNP and NPN input stage in which the PNP
stage senses common mode voltage near V
stage senses common mode voltage near V
from the PNP stage to NPN stage occurs 1V below V
both input stages have their own offset voltage, the offset of
the amplifier becomes a function of the input common mode
voltage and has a crossover point at 1V below V
in the V
FIGURE 3. Canceling the Voltage Offset Effect of Input
OS
vs. V
CM
curves.
to mid-supply). Below 1.2V the output
Bias Current
FIGURE 4.
DS100979-D2
+
−
. The transition
and the NPN
DS100979-59
www.national.com
+
as shown
+
. Since