lmv301-mdc National Semiconductor Corporation, lmv301-mdc Datasheet - Page 12

• Manufacturer Part Number: lmv301-mdc
• Description: Low Input Bias Current, 1.8v Op Amp W/ Rail-to-rail Output
• Manufacturer: National Semiconductor Corporation

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Application Hints
Compensating Input Capacitance
The high input resistance of the LMV301 op amp allows the
use of large feedback and source resistor values without
losing gain accuracy due to loading. However, the circuit will
be especially sensitive to its layout when these large value
resistors are used.
Every amplifier has some capacitance between each input
and AC ground, and also some differential capacitance
between the inputs. When the feedback network around an
amplifier is resistive, this input capacitance (along with any
additional capacitance due to circuit board traces, the
socket, etc.) and the feedback resistors create a pole in the
feedback path. In the following General Operational Amplifier
circuit, Figure 1 , the frequency of this pole is
where C
including amplifier input capacitance and any stray
capacitance from the IC socket (if one is used), circuit board
traces, etc., and R
This 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 k , the
frequency of the feedback pole will be quite high, since C
generally less than 10pF. 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” −3dB frequency, a feedback capacitor,
C
inverting input of the op amp. This condition can also be
stated in terms of the amplifier’s low frequency noise gain. To
maintain 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:
F
, should be connected between the output and the
S
is the total capacitance at the inverting input,
P
is the parallel combination of R
F
and R
S
S
IN
is
),
.
12
the following value of feedback capacitor is recommended:
If
the feedback capacitor should be:
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 capacitor will give much higher bandwidth
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
capacitance, or to tolerate additional phase shifts in the loop,
or excessive capacitive load, or to decrease the noise or
bandwidth, or simply because the particular circuit
implementation needs more feedback capacitance to be
sufficiently stable. For example, a printed circuit board’s
stray capacitance may be larger or smaller than the
breadboard’s, so the actual optimum value for C
different from the one estimated using the breadboard. In
most cases, the values of C
actual circuit, starting with the computed value.
S
S
consists of the amplifier’s input capacitance plus any stray capacitance
and the feedback resistors.
FIGURE 1. General Operational Amplifier Circuit
F
compensates for the pole caused by
F
should be checked on the
20019306
F
may be