Datasheet

+
-
V
OUT
V
IN
R
ISO
C
L
LMV841, LMV842, LMV844
SNOSAT1G –OCTOBER 2006–REVISED FEBRUARY 2013 www.ti.com
CAPACITIVE LOAD
The LMV841/LMV842/LMV844 can be connected as non-inverting unity gain amplifiers. This configuration is the
most sensitive to capacitive loading. The combination of a capacitive load placed on the output of an amplifier
along with the amplifier’s output impedance creates a phase lag, which reduces 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 LMV841/LMV842/LMV844 can directly drive capacitive loads up to 100pF without any stability issues. In
order to drive heavier capacitive loads, an isolation resistor, R
ISO
, should be used, as shown in Figure 38. By
using this isolation resistor, the capacitive load is isolated from the amplifier’s output, and hence, the pole caused
by C
L
is no longer in the feedback loop. The larger the value of R
ISO
, the more stable the output voltage will be. If
values of R
ISO
are sufficiently large, the feedback loop will be stable, independent of the value of C
L
. However,
larger values of R
ISO
result in reduced output swing and reduced output current drive.
Figure 38. Isolating Capacitive Load
DECOUPLING AND LAYOUT
For decoupling the supply lines it is suggested that 10nF capacitors be placed as close as possible to the op
amp.
For single supply, place a capacitor between V
+
and V
−
. For dual supplies, place one capacitor between V
+
and
the board ground, and the second capacitor between ground and V
−
.
OP AMP CIRCUIT NOISE
The LMV841/LMV842/LMV844 have good noise specifications, and will frequently be used in low-noise
applications. Therefore it is important to determine the noise of the total circuit. Besides the input referred noise
of the op amp, the feedback resistors may have an important contribution to the total noise.
For applications with a voltage input configuration it is, in general, beneficial to keep the resistor values low. In
these configurations high resistor values mean high noise levels. However, using low resistor values will increase
the power consumption of the application. This is not always acceptable for portable applications, so there is a
trade-off between noise level and power consumption.
Besides the noise contribution of the signal source, three types of noise need to be taken into account for
calculating the noise performance of an op amp circuit:
• Input referred voltage noise of the op amp
• Input referred current noise of the op amp
• Noise sources of the resistors in the feedback network, configuring the op amp
To calculate the noise voltage at the output of the op amp, the first step is to determine a total equivalent noise
source. This requires the transformation of all noise sources to the same reference node. A convenient choice for
this node is the input of the op amp circuit. The next step is to add all the noise sources. The final step is to
multiply the total equivalent input voltage noise with the gain of the op amp configuration.
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