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

Ω, 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

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

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:

Note that these capacitor values are usually significantly

smaller than those given by the older, more conservative

formula:

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.

20019306

FIGURE 1. General Operational Amplifier Circuit

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