LMP7701MASLASHNOPB datasheet(23/44 Pages) TI1

23 / 44 page

10k

100k

10M

FREQUENCY (Hz)

-10

-8

-6

-4

-2

VS = 5V

R1 = R2 = 100 k:

AV = -1

CF = 5 pF

CF = 3 pF

CF = 1 pF

CF = 0 pF

(1 - AV)

2A0AVCIN

10k

100k

10M

FREQUENCY (Hz)

-10

-8

-6

-4

-2

VS = 5V

CF = 0 pF

AV = -1

R1 = R2 = 100 k:

R1 = R2 = 30 k:

R1 = R2 = 10 k:

R1 = R2 = 1 k:

LMP7701, LMP7702, LMP7704

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SNOSAI9I – SEPTEMBER 2005 – REVISED NOVEMBER 2015

Feature Description (continued)

As shown in Equation 2, as values of R1 and R2 are increased, the magnitude of the poles is reduced, which in

turn decreases the bandwidth of the amplifier. Whenever possible, it is best to choose smaller feedback resistors.

Figure 43 shows the effect of the feedback resistor on the bandwidth of the LMP770x.

Figure 43. Closed-Loop Gain vs Frequency

Equation 2 has two poles. In most cases, it is the presence of pairs of poles that causes gain peaking. To

eliminate this effect, the poles should be placed in Butterworth position, because poles in Butterworth position do

not cause gain peaking. To achieve a Butterworth pair, the quantity under the square root in Equation 2 should

be set to equal

−1. Using this fact and the relation between R1 and R2, R2 = −AV R1, the optimum value for R1

can be found. This is shown in Equation 3. If R1 is chosen to be larger than this optimum value, gain peaking will

occur.

(3)

In Figure 42, CF is added to compensate for input capacitance and to increase stability. Additionally, CF reduces

or eliminates the gain peaking that can be caused by having a larger feedback resistor. Figure 44 shows how CF

reduces gain peaking.

Figure 44. Closed-Loop Gain vs Frequency With Compensation

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