LTC1264CS Linear Technology, LTC1264CS Datasheet - Page 11

• Manufacturer Part Number: LTC1264CS
• Description: High Speed/ Quad Universal Filter Building Block
• Manufacturer: Linear Technology

A
For example, for an LTC1264 bandpass filter with f
= 100kHz and f
produce a 100kHz, 10mV output. A 1st or 2nd order
prefilter will reduce aliasing to acceptable levels in most
cases.
A GUIDE TO BANDPASS DESIGN
Filter design tools like FCAD require design specification
inputs such as passband ripple, attenuation, passband
width and stopband width in order to calculate filter
parameters f
these filter approximations most often require Q values
which make excessive demands on the gain-bandwidth
products of active filter realizations. The active filter de-
signer should define a gain response so that the filter’s
mathematical approximation has practical requirements.
Table 4 is a guide to practical design specifications for
realizing bandpass filters with LTC1264 (please also refer
to the Typical Maximum Q vs Clock Frequency and Band-
pass Gain Error graphs under Typical Performance Char-
acteristics).
A Bandpass Design Example
Implementing the Bandpass Design
With the LTC1264 in Mode 1b, Butterworth and Chebyshev
bandpass designs with f
20:1 are possible.
First choose the clock frequency which in Mode 1b must
be greater than 20 times the bandpass center frequency of
40kHz. For this example, let’s choose f
Table 6 lists the resistors for for the bandpass design
example and Figure 11 shows the complete circuit.
PPLICATI
Filter Type:
Filter Response:
Passband Ripple:
Attenuation:
Center Frequency:
Passband Width:
Stopband Width:
O
, Q, f
CLK
O
n
U
= 2MHz, a 3.9MHz, 10mV input will
or poles and zeroes. The results of
S
CLK
Bandpass
Butterworth
3dB
60dB
40kHz (f
10kHz
60kHz
I FOR ATIO
U
to f
CENTER
CENTER
W
ratios greater than
CLK
)
to be 1MHz.
U
CENTER
Table 4. Bandpass Design Specifications (f
frequency of passband.)
Note: Reducing passband ripple or attenuation will decrease Q values. The
filter order may also increase.
Table 5. Calculated Filter Parameters
Table 6. Calculated Mode 1b Resistors to Nearest 1% Value
Using Table 5 Filter Parameters and Figure 10 Equations
0.1 for Chebyshev
STAGE
3dB for Butterworth
STAGE
PASSBAND
1
2
3
4
Figure 10. Equations for Resistors in Mode 1b Operation
RIPPLE
1
2
3
4
(dB)
52.3k
47.5k
56.2k
44.2k
R1
H
OBP
R2 = 10k
R5 = 5k
R1 =
R6 =
R3 =
f
i
=
=
PASSBAND
f
H
f
CLK
20
f
f
R3
R5•f
38.1201kHz
41.9726kHz
35.6418kHz
44.8911kHz
OBP
i
Q
2
CENTER
CENTER
WIDTH
10k
10k
10k
10k
R2
R6 + 5
– f
2
R2•Q
(Hz)
R6
O
O
(FOR BANDPASS)
2
f
2
f
CENTER
O
f
/20
/20
O
56.2k
51.1k
147k
118k
–
R3
f
CENTER
5 Passband –40 to –60
5 Passband –40 to –60
STOPBAND
f
O
WIDTH
CENTER
(Hz)
1264 F10
2
+ 1
LTC1264
R5
5k
5k
5k
5k
is center
10.5221
10.5221
4.3346
4.3346
Q
ATTENU-
11
ATION
(dB)
6.98k
11.8k
5.11k
20.5k
R6