![](http://datasheet.mmic.net.cn/Semtech/XE8805AMI028LF_datasheet_100002/XE8805AMI028LF_130.png)
Semtech 2006
www.semtech.com
18-9
XE8805/05A
XE88xx
DAS_OUT
DAS_AI_M
DAS_AO
DAS_AI_P
amp
analog ground
R1
R2
D
A
c
o
n
t
r
o
l
C
Figure 18-2. First order low pass filter.
18.6.2
Second order low pass filter
Figure 18-3 shows an example of a second order low pass filter using the multi-feedback architecture. The gain G,
cut-off frequency fc and the damping factor ξ (or quality factor Q) as a function of the factors k and m (see Figure
18-3) are given by:
kmn
RC
f
n
km
n
Q
k
G
c
π
ξ
2
1
2
)
1
(
2
1
=
+
=
=
For a second order Butterworth filter,
2
=
ξ
. For smaller damping factors, the filter is under damped resulting
in overshoots on the step response. For higher damping factors, the filter is over damped resulting in a smooth but
slower step response.
An example of a 1dB ripple Chebychev filter with a cut-off frequency of about 1.5kHz and a DC gain of 1 is given
by choosing m=0.22, k=1, n=0.5, R=330k
and C=1nF. The resistor nR can be rounded to 180k.
A 60Hz unity gain low pass Butterworth filter can be built choosing R=180k
, C=12nF, k=1, m=0.183, n=8.33.
Note that parasitic capacitors between the DAS_OUT node and the filter output DAS_AO will adversely affect the
high frequency behavior of the filter. Care should be taken when routing these signals.
Not
Recommended
for
New
Designs