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
S
= 3.3V, 5.0V
AC CMRR
DC CMRR
100
80
60
40
20
LMV851, LMV852, LMV854
www.ti.com SNOSAW1A –OCTOBER 2007–REVISED MARCH 2013
The configuration is largely the usually applied balanced configuration. With potentiometer P1, the balance can
be tuned to compensate for the DC offset in the DUT. The main difference is the addition of the buffer. This
buffer prevents the open-loop output impedance of the DUT from affecting the balance of the feedback network.
Now the closed-loop output impedance of the buffer is a part of the balance. But as the closed-loop output
impedance is much lower, and by careful selection of the buffer also has a larger bandwidth, the total effect is
that the CMRR of the DUT can be measured much more accurately. The differences are apparent in the larger
measured bandwidth of the AC CMRR.
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 50 shows a combination of the AC CMRR and the DC CMRR.
Figure 50. 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 7 mV from the positive and negative rail
The LMV851/LMV852/LMV854 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
LMV851/LMV852/LMV854 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 51. 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 51. Isolating Capacitive Load
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