Q =

1

jZ

2

(jZ)

2

+ 1

+

H(s) =

s

2

+ s *

1

1

+

+

+

1

1

OUT

+

-

Preamplifier

+IN

-IN

Output Buffer

Gain = K2

A1

A2

Gain = K1

OFFSET

Internal

100 k:

8

1

3

4

5

7

Level

shift

LMP8602, LMP8602Q, LMP8603, LMP8603Q

www.ti.com

SNOSB36D – JULY 2009 – REVISED MARCH 2013

ADDITIONAL SECOND ORDER LOW PASS FILTER

The LMP8602/LMP8602Q/LMP8603/LMP8603Q has a third order Butterworth low-pass characteristic with a

typical bandwidth of 60 kHz integrated in the preamplifier stage of the part. 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 as shown in Figure 43 by

second order low-pass filter characteristic.

K1 = 10, K2 = 5 for LMP8602, K2 = 10 for LMP8603

Figure 43. 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

•

•

(1)

(2)

The Cutt-off frequency

(3)

And the quality factor of the filter is given by:

(4)

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