Datasheet

+
-
V
OUT
V
IN
R
ISO
C
L
FREQUENCY (Hz)
CMRR (dB)
100
80
60
40
20
100 1k 10k 100k 1M 10M
V
+
= 3.3V, 5.0V
AC CMRR
DC CMRR
LMV861, LMV862
SNOSAZ5C –FEBRUARY 2008–REVISED MARCH 2013 www.ti.com
One artifact from this test circuit is that the low frequency CMRR results appear higher than expected. This is
because in the AC CMRR test circuit the potentiometer is used to compensate for the DC mismatches. So,
mainly AC mismatch is all that remains. Therefore, the obtained DC CMRR from this AC CMRR test circuit tends
to be higher than the actual DC CMRR based on DC measurements.
The CMRR curve in Figure 45 shows a combination of the AC CMRR and the DC CMRR.
Figure 45. CMRR Curve
OUTPUT CHARACTERISTICS
As already mentioned the output is rail-to-rail. When loading the output with a 10 kΩ resistor the maximum swing
of the output is typically 3 mV from the positive and negative rail.
The output of the LMV861 and LMV862 can drive currents up to 70 mA at 3.3V, and even up to 150 mA at 5V.
The LMV861 and LMV862 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 at 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 LMV861 and LMV862 can
directly drive capacitive loads up to 200 pF without any stability issues. In order to drive heavier capacitive loads,
an isolation resistor, R
ISO
, should be used, as shown in Figure 46. 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 amplifier will be. If the value of R
ISO
is 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 46. Isolating Capacitive Load
A resistor value of around 50Ω would be sufficient. As an example some values are given in the following table,
for 5V and an open loop gain of 111 dB.
C
LOAD
R
ISO
300 pF 62Ω
400 pF 55Ω
500 pF 50Ω
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