Datasheet

C
1
=
9Q
R
1
Z
0
=
1
C
4Q
01
ZR
C
1
=
(K
2

1)Q
R
1
Z
0
R
2
=
9Q
2
R
1
=
2
R
1
R
2
4Q
2
R
=
1
R
( )
2
1 QK
-
Q =
R
1
R
2
C
1
2
C
1
R
2
R
2
=
R
1
R
2
K
2

1 K
2
 1
Q =
R
1
R
2
C
1
2
R
1
C
1
+ R
2
C
1

(K
2
 1)R
1
C
1
K
2

1
K
2

1
C
2
=
C
1
K
2

1
LMP8602, LMP8602Q, LMP8603, LMP8603Q
SNOSB36D –JULY 2009–REVISED MARCH 2013 www.ti.com
For any filter gain K > 1x, the design procedure can be very simple if the two capacitors are chosen to in a
certain ratio.
(5)
Inserting this in the above equation for Q results in:
(6)
Which results in:
(7)
In this case, given the predetermined value of R1 = 100 kΩ (the internal resistor), the quality factor is set solely
by the value of the resistor R
2
.
R
2
can be calculated based on the desired value of Q as the first step of the design procedure with the following
equation:
(8)
For the gain of 5 for the LMP8602 this results in:
(9)
For the gain of 10 for the LMP8603 this is:
(10)
For instance, the value of Q can be set to 0.5√2 to create a Butterworth response, to 1/√3 to create a Bessel
response, or a 0.5 to create a critically damped response. Once the value of R
2
has been found, the second and
last step of the design procedure is to calculate the required value of C to give the desired low-pass cut-off
frequency using:
(11)
Which for the gain = 5 will give:
(12)
and for the gain = 10:
(13)
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