Datasheet

© 2008 Microchip Technology Inc. DS21811E-page 17
MCP6281/1R/2/3/4/5
FIGURE 4-15: Second-Order Multiple
Feedback Low-Pass Filter with an Extra
Pole-Zero Pair.
4.9.3.7 Second-Order Sallen-Key Low-Pass
Filter with an Extra Pole-Zero Pair
Figure 4-16 is a second-order Sallen-Key low-pass
filter with Chip Select. Use the FilterLab
®
software from
Microchip to determine the R and C values for the op
amp A’s second-order filter. Op amp B can be used to
add a pole-zero pair using C
3
, R
5
and R
6
.
FIGURE 4-16: Second-Order Sallen-Key
Low-Pass Filter with an Extra Pole-Zero Pair and
Chip Select.
4.9.3.8 Capacitorless Second-Order
Low-Pass filter with Chip Select
The low-pass filter shown in Figure 4-17 does not
require external capacitors and uses only three exter-
nal resistors; the op amp's GBWP sets the corner
frequency. R
1
and R
2
are used to set the circuit gain
and R
3
is used to set the Q. To avoid gain peaking in
the frequency response, Q needs to be low (lower
values need to be selected for R
3
). Note that the ampli-
fier bandwidth varies greatly over temperature and
process. However, this configuration provides a low-
cost solution for applications with high bandwidth
requirements.
FIGURE 4-17: Capacitorless Second-Order
Low-Pass Filter with Chip Select.
A
B
CS
R
1
C
1
R
5
V
IN
V
OUT
C
2
R
4
R
3
R
2
R
6
C
3
MCP6285
R
7
A
B
CS
R
2
C
1
R
1
V
IN
V
OUT
R
4
R
3
C
2
C
3R
5
MCP6285
R
6
A
B
CS
V
REF
V
IN
V
OUT
R
2
R
1
R
3
MCP6285