18
LTC1563-2/LTC1563-3
156323fa
TYPICAL APPLICATIO S
U
50kHz Wideband Bandpass
4th Order Bessel Lowpass at 128kHz with Two Highpass Poles at 11.7kHz Yields a Wideband Bandpass Centered at 50kHz
To design these wideband bandpass filters with the
LTC1563, start with a 4th order lowpass filter and add two
highpass poles with the input, AC coupling capacitors. The
lowpass cutoff frequency and highpass pole frequencies
depend on the specific application. Some experimentation
of lowpass and highpass frequencies is required to achieve
the desired response. FilterCAD does not directly support
this configuration. Use the custom design window in
FilterCAD get the desired response and then use FilterCAD
to give the schematic for the lowpass portion of the filter.
Calculate the two highpass poles using the following
formulae:
f HPA
RC
f HPB
RC
OO
()
=
()
=
1
21111
1
21212•• •
,
•• •ππ
The design process is as follows:
1. After starting FilterCAD, select the Enhanced Design
window.
2. Choose a 4th order Bessel or Butterworth lowpass filter
response and set the cutoff frequency to the high
frequency corner of the desired bandpass.
3. Click on the custom response button. This copies the
lowpass coefficients into the custom design Coeffi-
cients table.
4. In the Coefficients table, the first two rows are the LP
Type with the f
O
and Q as previously defined. Go to the
third and fourth rows and click on the Type column
(currently a hyphen is in this space). Change the Type
of each of these rows to type HP1. This sets up a
template of a 6th order filter with two 2nd order lowpass
sections and two 1st order highpass sections.
5. Change the frequency of the highpass (HP1) poles to
get the desired frequency response.
6. You may have to perform this loop several times before
you close in on the correct response.
7. Once you have reached a satisfactory response, note
the highpass pole frequencies. The HP1 highpass poles
must now be removed from the Custom design coeffi-
cients table. After removing the highpass poles, click on
the Implement button to go on to the filter implementa-
tion stage.
8. In the Enhanced Implement window, click on the Active
RC button and choose the LTC1563-2 part. Click on the
schematic button to view the resulting circuit.
9. You now have the schematic for the 4th order lowpass
part of the design. Now calculate the capacitor values
from the following formulae:
C
R f HPA
C
R f HPB
OO
11
1
211
12
1
212
=
()
=
()
•• •
,
•• •ππ
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
LP
SA
NC
INVA
NC
LPA
AGND
V
–
V
+
LPB
NC
INVB
NC
SB
NC
EN
V
OUT
5V
V
IN
1563 TA09
R31
20k
R32
20k
R22
20k
R21
20k
R11
20k
R12
20k
0.1µF
LTC1563-3
0.1µF
C12
680pF
C11
680pF
–5V
10
0
–10
–20
–30
–40
–50
–60
1k 10k 100k 1M
FREQUENCY (Hz)
GAIN (dB)
1563 TA09a
