LMP8602MASLASHNOPB datasheet(20/42 Pages) TI1

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R1R2C1C2

Zo =

QZo

(jZ)

+ 1

G(jZ) = K1 *

H(s) =

+ s *

R1R2C1C2

R1C2

R2C2

K1 * K2

(1 - K2)

R2C1

R1R2C1C2

OUT

Preamplifier

+IN

-IN

Output Buffer

Gain = K2

Gain = K1

OFFSET

Internal

100 k:

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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Feature Description (continued)

7.3.2 Additional Second-Order Low-Pass Filter

The LMP86x1 and LMP86x1-Q1 have a third-order Butterworth lowpass characteristic with a typical bandwidth of

60 kHz integrated in the preamplifier stage. The bandwidth of the output buffer can be reduced by adding a

capacitor on the A1 pin to create a first-order low-pass filter with a time constant determined by the 100-k

Ω

internal resistor and the external filter capacitor.

It is also possible to create an additional second-order, Sallen-Key, low-pass filter by adding external

components R2, C1 and C2. Together with the internal 100-kΩ resistor R1 as illustrated in Figure 53, this circuit

creates a second-order, low-pass filter characteristic.

NOTE: K1 = 10; K2 = 2 for LMP8601, LMP8601-Q1; 5 for LMP8602, LMP8602-Q1; or 10 for LMP8603, LMP8603-Q1.

Figure 53. Second-Order Low-Pass Filter

When the corner frequency of the additional filter is much lower than 60 kHz, the transfer function of the

described amplifier can be written as:

where

•

K1 equals the gain of the preamplifier and K2 that of the buffer amplifier.

(1)

Equation 1 can be written in the normalized frequency response for a second-order lowpass filter:

(2)

The cutoff frequency

ωo in rad/sec (divide by 2π to get the cut-off frequency in Hz) is given by:

(3)

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