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
LMV831, LMV832, LMV834
www.ti.com SNOSAZ6B –AUGUST 2008–REVISED MARCH 2013
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 48 shows a combination of the AC CMRR and the DC CMRR.
Figure 48. 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 6 mV from the positive and negative rail.
The output of the LMV831/LMV832/LMV834 can drive currents up to 30 mA at 3.3V and even up to 65 mA at 5V
The LMV831/LMV832/LMV834 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
LMV831/LMV832/LMV834 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 49. 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 49. Isolating Capacitive Load
A resistor value of around 150Ω would be sufficient. As an example some values are given in the following table,
for 5V.
C
LOAD
R
ISO
300 pF 165Ω
400 pF 175Ω
500 pF 185Ω
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