Datasheet
Q =
R
1
C
1
+ R
2
C
1
+ (1 - K
2
) * R
1
C
2
R
1
R
2
C
1
C
2
1
R
1
R
2
C
1
C
2
Z
o
=
K
2
QZ
o
jZ
Z
o
2
(jZ)
2
+ 1
+
G(jZ) = K
1
*
H(s) =
s
2
+ s *
R
1
R
2
C
1
C
2
1
1
R
1
C
2
+
R
2
C
2
+
+
K
1
* K
2
(1 - K
2
)
R
2
C
1
1
R
1
R
2
C
1
C
2
1
OUT
+
-
Preamplifier
+IN
-IN
Output Buffer
Gain = K2
A1 A2
Gain = K1
OFFSET
Internal
R
1
100 k:
C
2
C
1
R
2
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 kΩ 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
adding external components R
2
, C
1
and C
2
. Together with the internal 100 kΩ resistor R
1
, this circuit creates a
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
• K
1
equals the gain of the preamplifier
• K
2
that of the buffer amplifier (1)
The above equation can be written in the normalized frequency response for a 2
nd
order low pass filter:
(2)
The Cutt-off frequency ω
o
in rad/sec (divide by 2π to get the cut-off frequency in Hz) is given by:
(3)
And the quality factor of the filter is given by:
(4)
Copyright © 2009–2013, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LMP8602 LMP8602Q LMP8603 LMP8603Q