LMP8278 datasheet(12/23 Pages) TI

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Ri = 100 k: x

Gi - 14

Gi =

Ri - 100 k:

14 Ri

Rr = 100 k: x

14 - Gr

Gr =

Rr + 100 k:

14 Rr

LMP8278

SNAS575A – FEBRUARY 2012 – REVISED MARCH 2013

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Note that the frequency response achieved using this procedure will only be accurate if the cut-off frequency of

the second order filter is much smaller than the intrinsic 90 kHz low-pass response. In other words, choose the

frequency response of the circuit such that the internal poles of the LMP8278 do not affect the external second

order filter.

For a desired Q = 0.707 and a cut off frequency = 3 kHz, this will result in rounded values for R2 = 200 kΩ, C1:C2

= 390 pF.

GAIN ADJUSTMENT

The gain of the LMP8278 is 14; however, this gain can be adjusted as the signal path in between the two internal

amplifiers is available on the external pins.

Reduce Gain

Figure 22 shows the configuration that can be used to reduce the gain of the LMP8278.

Rr creates a resistive divider together with the internal 100 kΩ resistor such that the reduced gain Gr becomes:

(12)

Given a desired value of the reduced gain Gr, using this equation the required value for Rr can be calculated

with:

(13)

Increase Gain

Figure 23 shows the configuration that can be used to increase the gain of the LMP8278.

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 becomes:

(14)

From this equation, for a desired value of the gain, the required value of Ri can be calculated with:

(15)

It should be noted from the equation for the gain Gi that 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 will be

determined by tolerances in the values 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 will even become

unstable. It is recommended to limit the application of this technique to gain values of 35 or smaller.

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