XE8805 SEMTECH [Semtech Corporation], XE8805 Datasheet - Page 130

• Manufacturer Part Number: XE8805
• Description: Data Acquisition MCU
• Manufacturer: SEMTECH [Semtech Corporation]

18.6.2
Figure 18-3 shows an example of a second order low pass filter using the multi-feedback architecture. The gain G,
cut-off frequency f
18-3) are given by:
For a second order Butterworth filter,
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=330kΩ and C=1nF. The resistor nR can be rounded to 180kΩ.
A 60Hz unity gain low pass Butterworth filter can be built choosing R=180kΩ, 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.
© Semtech 2006
XE88xx
ξ
G
f
c
=
Second order low pass filter
=
=
2
−
1
2
Q
k
π
c
o
n
o
r
t
l
RC
=
c
(
and the damping factor ξ (or quality factor Q) as a function of the factors k and m (see Figure
1
n
kmn
+
2
) 1
D
n
km
A
ξ
Figure 18-2. First order low pass filter.
=
amp
2
2
. For smaller damping factors, the filter is under damped resulting
DAS_AI_M
18-9
DAS_AI_P
DAS_OUT
DAS_AO
analog ground
R
R
1
2
C
XE8805/05A
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