lmc6034-mda National Semiconductor Corporation, lmc6034-mda Datasheet - Page 7

• Manufacturer Part Number: lmc6034-mda
• Description: Cmos Quad Operational Amplifier
• Manufacturer: National Semiconductor Corporation

Applications Hint
etc., and R
formula, as well as all formulae derived below, apply to
inverting and non-inverting op-amp configurations.
When the feedback resistors are smaller than a few kΩ, the
frequency of the feedback pole will be quite high, since C
generally less than 10 pF. If the frequency of the feedback
pole is much higher than the “ideal” closed-loop bandwidth
(the nominal closed-loop bandwidth in the absence of C
the pole will have a negligible effect on stability, as it will add
only a small amount of phase shift.
However, if the feedback pole is less than approximately 6 to
10 times the “ideal” −3 dB frequency, a feedback capacitor,
C
ing input of the op amp. This condition can also be stated in
terms of the amplifier’s low-frequency noise gain: To main-
tain stability a feedback capacitor will probably be needed if
where
is the amplifier’s low-frequency noise gain and GBW is the
amplifier’s gain bandwidth product. An amplifier’s low-
frequency noise gain is represented by the formula
regardless of whether the amplifier is being used in inverting
or non-inverting mode. Note that a feedback capacitor is
more likely to be needed when the noise gain is low and/or
the feedback resistor is large.
If the above condition is met (indicating a feedback capacitor
will probably be needed), and the noise gain is large enough
that:
the following value of feedback capacitor is recommended:
If
the feedback capacitor should be:
F
, should be connected between the output and the invert-
P
is the parallel combination of R
(Continued)
F
and R
IN
. This
S
S
is
),
7
Note that these capacitor values are usually significantly
smaller than those given by the older, more conservative
formula:
C
from the circuit board and socket. C
C
Using the smaller capacitors will give much higher band-
width with little degradation of transient response. It may be
necessary in any of the above cases to use a somewhat
larger feedback capacitor to allow for unexpected stray ca-
pacitance, or to tolerate additional phase shifts in the loop, or
excessive capacitive load, or to decrease the noise or band-
width, or simply because the particular circuit implementa-
tion needs more feedback capacitance to be sufficiently
stable. For example, a printed circuit board’s stray capaci-
tance may be larger or smaller than the breadboard’s, so the
actual optimum value for C
estimated using the breadboard. In most cases, the values
of C
the computed value.
Capacitive Load Tolerance
Like many other op amps, the LMC6034 may oscillate when
its applied load appears capacitive. The threshold of oscilla-
tion varies both with load and circuit gain. The configuration
most sensitive to oscillation is a unity-gain follower. See
Typical Performance Characteristics.
The load capacitance interacts with the op amp’s output
resistance to create an additional pole. If this pole frequency
is sufficiently low, it will degrade the op amp’s phase margin
so that the amplifier is no longer stable at low gains. As
shown in Figure 3, the addition of a small resistor (50Ω to
100Ω) in series with the op amp’s output, and a capacitor (5
pF to 10 pF) from inverting input to output pins, returns the
phase margin to a safe value without interfering with lower-
frequency circuit operation. Thus larger values of capaci-
tance can be tolerated without oscillation. Note that in all
cases, the output will ring heavily when the load capacitance
is near the threshold for oscillation.
S
S
consists of the amplifier’s input capacitance plus any stray capacitance
and the feedback resistors.
FIGURE 2. General Operational Amplifier Circuit
F
should be checked on the actual circuit, starting with
F
F
may be different from the one
compensates for the pole caused by
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