LMP7721

SNOSAW6E – JANUARY 2008 – REVISED DECEMBER 2014

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Device Functional Modes (continued)

In the General Operational Amplifier circuit, Figure 45 the frequency of this pole is:

(1)

where:

•

capacitance from the circuit board traces.

•

The typical input capacitance of the LMP7721 is about 11pF. This formula, as well as all formulas 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,

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

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 noninverting 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)

If

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