Datasheet

16
LTC1563-2/LTC1563-3
156323fa
TYPICAL APPLICATIO S
U
100kHz, 6th Order Pseudo-Butterworth
Frequency Response
TEXTBOOK BUTTERWORTH PSEUDO-BUTTERWORTH
f
O
1 = 100kHz Q1 = 1.9319 f
O
1 = 100kHz Q1 = 1.9319
f
O
2 = 100kHz Q2 = 0.7071 f
O
2 = 100kHz Q2 = 0.7358
f
O
3 = 100kHz Q3 = 0.5176 f
O
3 = 100kHz Real Poles
f
O
4 = 100kHz Real Poles
The complex, 2nd order section of the textbook design
with the lowest Q is replaced with two real first order poles.
The Q of another section is slightly altered such that the
final filter’s response is indistinguisable from a textbook
Butterworth response.
The f
O
and Q values listed above can be entered in
FilterCAD’s Enhanced Design window as a custom re-
sponse filter. After entering the coefficients, FilterCAD will
produce a schematic of the circuit. The procedure is as
follows:
1. After starting FilterCAD, select the Enhanced Design
window.
2. Select the Custom Response and set the custom F
C
to
1Hz.
3. In the Coefficients table, go to the Type column and click
on the types listed and set the column with two LP types
and two LP1 types. This sets up a template of a 6th order
filter with two 2nd order lowpass sections and two 1st
order lowpass sections.
4. Enter the f
O
and Q coefficients as listed above. For a
Butterworth filter, use the same coefficients as the
example circuit above except set all of the f
O
to 1Hz.
5. Set the custom F
C
to the desired cutoff frequency. This
will automatically multiply all of the f
O
coefficients. You
have now finished the design of the filter and you can
click on the frequency response or step response
buttons to verify the filter’s response.
6. Click on the Implement button to go on to the filter
implementation stage.
7. In the Enhanced Implement window, click on the Active
RC button to choose the LTC1563-2 part. You are now
done with the filter’s implementation. Click on the
schematic button to view the resulting circuit.
Other Pseudo Filter Response Coefficients (All f
O
Are Normalized for a 1Hz Filter Cutoff)
BESSEL 0.1dB RIPPLE CHEBYSHEV 0.5dB RIPPLE CHEBYSHEV TRANSITIONAL GAUSSIAN TO 12dB TRANSITIONAL GAUSSIAN TO 6dB
f
O
1 1.9070 1.0600 1.0100 2.1000 1.5000
Q1 1.0230 3.8500 5.3000 2.2000 2.8500
f
O
2 1.6910 0.8000 0.7200 1.2500 1.0500
Q2 0.6110 1.0000 1.2000 0.8000 0.9000
f
O
3 1.6060 0.6000 0.5000 1.2500 0.9000
f
O
4 1.6060 1.0000 0.8000 1.2500 0.9000
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
3.3V
V
IN
1563 TA07
R31
17.8k
R32
20.5k
R22
28.7k
R21
32.4k
0.1µF
LTC1563-2
0.1µF
C12
560pF
C11
560pF
R
A2
3.16k R
B2
25.5k
R
A1
3.16k R
B1
29.4k
FREQUENCY (Hz)
10k
GAIN (dB)
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
100k 1M
1563 TA07a