LMP7702 datasheet(17/35 Pages) TI

17 / 35 page

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:

-1

2CIN

P1,2 =

4 A0CIN

-R2/R1

1 +

CIN R2

VOUT

VIN

(s) =

A0 R1

R1 + R2

CIN

VOUT

VIN

VOUT

VIN

AV = -

= -

LMP7701, LMP7702, LMP7704

www.ti.com

SNOSAI9H – SEPTEMBER 2005 – REVISED MARCH 2013

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

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 will also cause gain peaking. In order to compensate for the input

capacitance, care must be taken in choosing the 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 46 is simply –R2/R1.

Figure 46. Compensating for Input Capacitance

For the time being, ignore CF. The AC gain of the circuit in Figure 46 can be calculated as follows:

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

(1)

As shown in Equation 1, 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 47 shows the effect of the feedback resistor on the bandwidth of the LMP7701/LMP7702/LMP7704.

Figure 47. Closed Loop Gain vs. Frequency

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