LMP7721

SNOSAW6E – JANUARY 2008 – REVISED DECEMBER 2014

www.ti.com

Device Functional Modes (continued)

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

(1)

where:

•

CS is the total capacitance at the inverting input, including amplifier input capacitance and any stray

capacitance from the circuit board traces.

•

RP is the parallel combination of RF and RIN

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,

since CS is generally less than 15 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 CS), 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, CF, 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 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

18

Submit Documentation Feedback

Copyright © 2008–2014, Texas Instruments Incorporated

Product Folder Links: LMP7721