LMP7715MFSLASHNOPB datasheet(15/27 Pages) TI

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-1

2CIN

P1,2 =

4 A0CIN

-R2/R1

1 +

CIN R2

VOUT

VIN

(s) =

A0 R1

R1 + R2

CIN

VOUT

VIN

VOUT

VIN

AV = -

= -

VCM (V)

VS = 5V

LMP7715, LMP7716, LMP7716Q

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SNOSAV0E – MARCH 2006 – REVISED MARCH 2013

Figure 50. Input Common Mode Capacitance

This input capacitance will interact with other impedances, such as gain and feedback resistors which are seen

on the inputs of the amplifier, to form a pole. This pole will have little or no effect on the output of the amplifier at

low frequencies and under DC conditions, but will play a bigger role as the frequency increases. At higher

frequencies, the presence of this pole will decrease phase margin and also cause gain peaking. In order to

compensate for the input capacitance, care must be taken in choosing feedback resistors. In addition to being

selective in picking values for the feedback resistor, a capacitor can be added to the feedback path to increase

stability.

The DC gain of the circuit shown in Figure 51 is simply

−R2/R1.

Figure 51. Compensating for Input Capacitance

For the time being, ignore CF. The AC gain of the circuit in Figure 51 can be calculated as follows:

(1)

This equation is rearranged to find the location of the two poles:

(2)

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

which in turn decreases the bandwidth of the amplifier. Figure 52 shows the frequency response with different

value resistors for R1 and R2. Whenever possible, it is best to chose smaller feedback resistors.

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