LMP8602MASLASHNOPB datasheet(24/42 Pages) TI1

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Ri = 900 k: x

Gi - 100

Ri = 400 k: X

Gi - 50

Ri = 100 k: X

Gi - 20

Gi =

Ri - 900 k:

100 Ri

Gi =

Ri - 400 k:

50 Ri

Gi =

Ri - 100 k:

20 Ri

+IN

-IN

Preamplifier

Output Buffer

Gain = K2

Gain = 10

OFFSET

Internal

Resistor

100 k:

OUT

Level

shift

LMP8601, LMP8601-Q1

LMP8602, LMP8602-Q1

LMP8603, LMP8603-Q1

SNOSAR2H – SEPTEMBER 2008 – REVISED APRIL 2016

www.ti.com

Product Folder Links: LMP8601 LMP8601-Q1 LMP8602 LMP8602-Q1 LMP8603 LMP8603-Q1

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Device Functional Modes (continued)

NOTE: K2 = 2 for LMP8601, LMP8601-Q1; 5 for LMP8602, LMP8602-Q1; or 10 for LMP8603, LMP8603-Q1.

Figure 55. Increase Gain

Ri creates positive feedback from the output pin to the input of the buffer amplifier. The positive feedback

increases the gain. The increased gain Gi for the LMP8601 and LMP8601-Q1 becomes:

(27)

For the LMP8602 and LMP8602-Q1:

(28)

And for the LMP8603 and LMP8603-Q1:

(29)

From this equation, for a desired value of the gain, the LMP8601 and LMP8601-Q1 required value of Ri is

calculated with:

(30)

For the LMP8602 and LMP8602-Q1:

(31)

And for the LMP8603 with:

(32)

Note that from the equation for the gain Gi, for large gains, Ri approaches 100 kΩ. In this case, the denominator

in the equation becomes close to zero. In practice, for large gains, the denominator is determined by tolerances

in the value of the external resistor Ri and the internal 100-kΩ resistor. In this case, the gain becomes very

inaccurate. If the denominator becomes equal to zero, the system becomes unstable. TI recommends to limit the

application of this technique to gain values of 50 or smaller.

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