11

LTC1264

S

APPLICATI

I FOR ATIO

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

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:

Bandpass

Filter Response:

Butterworth

Passband Ripple:

3dB

Attenuation:

60dB

Center Frequency:

Passband Width:

10kHz

Stopband Width:

60kHz

Implementing the Bandpass Design

With the LTC1264 in Mode 1b, Butterworth and Chebyshev

20:1 are possible.

First choose the clock frequency which in Mode 1b must

be greater than 20 times the bandpass center frequency of

Table 6 lists the resistors for for the bandpass design

example and Figure 11 shows the complete circuit.

frequency of passband.)

PASSBAND

PASSBAND

STOPBAND

ATTENU-

RIPPLE

WIDTH

WIDTH

ATION

(dB)

(Hz)

(Hz)

(dB)

≤ 3dB for Butterworth

≥ 5 × Passband –40 to –60

≤0.1 for Chebyshev

≥ 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

Q

1

38.1201kHz

4.3346

2

41.9726kHz

4.3346

3

35.6418kHz

10.5221

4

44.8911kHz

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

Figure 10. Equations for Resistors in Mode 1b Operation

R2 = 10k

R5 = 5k

R1 =

(FOR BANDPASS)

R6 =

+ 1

R3 =

R3

2

2

2

()

Q

2

–

R2•Q

R6

R6 + 5

()

√

1264 F10

20