LM6132/34 Application Hints
(Continued)
cellent response is obtained with a C
f
of 39 pF. In
Figure 4
,
the supplies have been reduced to
±
2.5V, the pulse is
4 Vp-p and C
f
is 39 pF. The best value for the compensation
capacitor should be established after the board layout is fin-
ished because the value is dependent on board stray capac-
ity, the value of the feedback resistor, the closed loop gain
and, to some extent, the supply voltage.
Another effect that is common to all opamps is the phase
shift caused by the feedback resistor and the input capaci-
tance. This phase shift also reduces phase margin. This ef-
fect is taken care of at the same time as the effect of the ca-
pacitive load when the capacitor is placed across the
feedback resistor.
The circuit shown in
Figure 5
was used for these scope
photos.
Figure 6
shows a method for compensating for load capaci-
tance (C
o
) effects by adding both an isolation resistor Ro at
the output and a feedback capacitor C
F
directly between the
output and the inverting input pin. Feedback capacitor C
F
compensates for the pole introduced by R
o
and C
o
, minimiz-
ing ringing in the output waveform while the feedback resis-
tor R
F
compensates for dc inaccuracies introduced by R
o
.
Depending on the size of the load capacitance, the value of
R
o
is typically chosen to be between 100Ω to1kΩ.
Typical Applications
3 OPAMP INSTRUMENTATION AMP WITH
RAIL-TO-RAIL INPUT AND OUTPUT
Using the LM6134, a 3 opamp instrumentation amplifier with
rail-to-rail inputs and rail to rail output can be made. These
features make these instrumentation amplifiers ideal for
single supply systems.
Some manufacturers use a precision voltage divider array of
5 resistors to divide the common-mode voltage to get an in-
put range of rail-to-rail or greater. The problem with this
method is that it also divides the signal, so to even get unity
gain, the amplifier must be run at high closed loop gains.
This raises the noise and drift by the internal gain factor and
lowers the input impedance. Any mismatch in these preci-
sion resistors reduces the CMR as well. Using the LM6134,
all of these problems are eliminated.
In this example, amplifiers A and B act as buffers to the dif-
ferential stage (
Figure 7
). These buffers assure that the input
impedance is over 100 MΩ and they eliminate the require-
ment for precision matched resistors in the input stage. They
also assure that the difference amp is driven from a voltage
source. This is necessary to maintain the CMR set by the
matching of R1–R2 with R3–R4.
DS012349-45
FIGURE 3.
DS012349-42
FIGURE 4.
DS012349-43
FIGURE 5.
DS012349-37
FIGURE 6.
LM6132/LM6134
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