LTC1264 Ver la hoja de datos (PDF) - Linear Technology
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LTC1264 Datasheet PDF : 16 Pages
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LTC1264
APPLICATI S I FOR ATIO
For example, for an LTC1264 bandpass filter with fCENTER
= 100kHz and fCLK = 2MHz, a 3.9MHz, 10mV input will
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 fO, Q, fn or poles and zeroes. The results of
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
Filter Type:
Filter Response:
Passband Ripple:
Attenuation:
Center Frequency:
Passband Width:
Stopband Width:
Bandpass
Butterworth
3dB
60dB
40kHz (fCENTER)
10kHz
60kHz
Implementing the Bandpass Design
With the LTC1264 in Mode 1b, Butterworth and Chebyshev
bandpass designs with fCLK to fCENTER ratios greater than
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 fCLK to be 1MHz.
Table 6 lists the resistors for for the bandpass design
example and Figure 11 shows the complete circuit.
Table 4. Bandpass Design Specifications (fCENTER is center
frequency of passband.)
PASSBAND
RIPPLE
(dB)
PASSBAND
WIDTH
(Hz)
STOPBAND
WIDTH
(Hz)
ATTENU-
ATION
(dB)
≤ 3dB for Butterworth
≤ 0.1 for Chebyshev
≥ fCENTER/20
≥ fCENTER/20
≥ 5 × Passband –40 to –60
≥ 5 × Passband –40 to –60
Note: Reducing passband ripple or attenuation will decrease Q values. The
filter order may also increase.
Table 5. Calculated Filter Parameters
STAGE
1
2
3
4
fO
38.1201kHz
41.9726kHz
35.6418kHz
44.8911kHz
Q
4.3346
4.3346
10.5221
10.5221
Table 6. Calculated Mode 1b Resistors to Nearest 1% Value
Using Table 5 Filter Parameters and Figure 10 Equations
STAGE
R1
R2
R3
R5
R6
1
52.3k
10k
56.2k
5k
6.98k
2
47.5k
10k
51.1k
5k
11.8k
3
56.2k
10k
147k
5k
5.11k
4
44.2k
10k
118k
5k
20.5k
R2 = 10k
R5 = 5k
fi =
fCLK
20
R1 = R3 (FOR BANDPASS)
HOBP
( ) R6 =
R5•fO2
fi2 – fO2
√ ( ) ( )2
HOBP =
Q2
fO
fCENTER
–
fCENTER
fO
+1
R3 = R2•Q
√ R6
(R6 + 5)
1264 F10
Figure 10. Equations for Resistors in Mode 1b Operation
1264fb
11
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