18

LTC1563-2/LTC1563-3

156323fa

TYPICAL APPLICATIO S

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

211

11

1

212

12

••

•

,

••

•

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

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

LPB

NC

INVB

NC

SB

NC

EN

5V

1563 TA09

R31

20k

R32

20k

R22

20k

R21

20k

R11

20k

R12

20k

0.1

µF

LTC1563-3

0.1

µF

C12

680pF

C11

680pF

–5V

10

0

–10

–20

–30

–40

–50

–60

1k

10k

100k

1M

FREQUENCY (Hz)

1563 TA09a