2

C =

1

C

K - 1

Q

(K-1)R C

1

2

R C + R C -

1

1

1

2

(K-1)

=

R R

1

2

2

1

C

(K-1)

Q =

1

jZ

2

(jZ)

2

+ 1

+

H(s) =

s

2

+ s *

1

1

+

+

+

1

1

LMP8278

SNAS575A – FEBRUARY 2012 – REVISED MARCH 2013

www.ti.com

ADDITIONAL SECOND ORDER LOW PASS FILTER

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 illustrated in Figure 21, by

second order low-pass filter characteristic.

When the corner frequency of the additional filter is much lower than 90 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)

(5)

For any filter gain K > 1x, the design procedure can be very simple if the two capacitors are chosen in a certain

ratio.

(6)

In this case, given the predetermined value of R1 = 100 k

Ω (the internal resistor), the quality factor is set solely

10

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