LMV301_14 Datasheet, PDF (13/26 Pages) National Semiconductor (TI) – LMV301 Low Input Bias Current, 1.8V Op Amp w/ Rail-to-Rail Output

LMV301
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APPLICATION HINTS
SNOS968A – MAY 2004 – REVISED MAY 2013
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 35, the
frequency of this pole is
where
• C_S is the total capacitance at the inverting input, including amplifier input capacitance and any stray
capacitance from the IC socket (if one is used), circuit board traces, etc.,
• R_P is the parallel combination of R_F and R_IN
(1)
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 kΩ, the frequency of the feedback pole will be quite high,
since C_S is 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_S), 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_F, should be connected between the output and the 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
(2)
Where
(3)
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
(4)
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:
(5)
the following value of feedback capacitor is recommended:
(6)
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