MAX260AENG Просмотр технического описания (PDF) - Maxim Integrated

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MAX260AENG

Microprocessor Programmable Universal Active Filters

Maxim Integrated 

Microprocessor Programmable

Universal Active Filters

since each offset is typical negative and each section

inverts. When the HP or BP outputs are used, the offset

can be removed with capacitor coupling.

Design Examples

Fourth-Order Chebyshev Bandpass Filter

Figure 22 shows both halves of a MAX260 cascaded to

form a fourth-order Chebyshev bandpass filter. The

desired parameters are:

Center frequency (f0)

= 1kHz

Pass bandwidth

= 200Hz

Stop bandwidth

= 600Hz

Max passband ripple

= 0.5dB

Min stopband attenuation = 15dB

From the previous parameters, the order (number of

poles) and the f0 and Q of each section can be deter-

mined. Such a derivation is beyond the scope of this

data sheet; however, there are a number of sources

that provide design data for this procedure. These

include look-up tables, design texts, and computer pro-

grams. Design software is available from Maxim to pro-

vide comprehensive solutions for most popular filter

configurations. The A and B section parameters for the

above filter are:

f0A = 904Hz

f0B = 1106Hz

QA = 7.05

QB = 7.05

To implement this filter, both halves operate in mode 1

and use the same clock. See Tables 2 and 3. The pro-

grammed parameters are:

CLKA = CLKB = 150kHz

fCLK/f0A = 166.50 (Mode 1, N = 42), actual f0A = 902.4Hz

fCLK/f0B = 136.66 (Mode 1, N = 23), actual f0B =

1099.7Hz

QA = QB = 7.11 (Mode 1, N = 119)

Sampling errors are very small at this fCLK/f0 ratio, so

the actual realized Q is very close to 7.05 (see Figure

20 or program MPP in the Filter Design Software sec-

tion). Often the realized Q is not exactly the target value

at high Qs because programming resolution lowers as

Q increases. This does not affect most filter designs,

since three-digit Q accuracy is practically never

required, and a Q resolution of 1 is provided up to Qs

of 10. The overall filter gain at f0 is 16.4V/V or 24.3dB

(see the Cascading Filters section). If another gain is

required, amplification or attenuation must be added at

the input, output, or between stages.

fO ERROR vs. fCLK/fO RATIO (MODE 1, 3)

20

18

f0 ERROR IS PLOTTED FOR MODES 1 AND 3

MODE 2: MULTIPLY ICLKIO BY √2 and

16

DIVIDE Q BY √2 BEFORE USING GRAPH

14

MODE 4: MUTIPLY fO ERROR BY 1.5

12

Q = 0.512

10

Q = 0.512

8

Q = 0.512

6

Q = 0.512 Q = 0.512

Q = 0.512

4

2

0

40 60 80 100 120 140 160 180 200

fCLK/fO RATIO

Q ERROR vs. fCLK/fO RATIO

-7

Q ERROR IS PLOTTED FOR MODES 1 AND 3

-6

MODE 2: MULTIPLY fCLK/fO BY √2 and

DIVIDE Q BY √2 BEFORE USING GRAPH

-5

MODE 4: MUTIPLY Q ERROR BY 1.5

Q = 0.5

-4

Q = 0.6

Q = 0.83

-3

Q = 1.21 Q = 3.05

-2

Q = 7.11

-1

0

40 60 80 100 120 140 160 180 200

fCLK/fO RATIO

Figure 20. Sampling Errors in fCLK/f0 and Q at Low fCLK/f0 and

Q Settings

VIN

+5V

R2 100kΩ

R3 270kΩ

100kΩ

-5V

OFFSET

TRIM

C1

R1 100kΩ

-

+

GAIN = -R1/R2

1

fLP = 2πR1C2

NOTE: OP AMP INCLUDED WITH MAX261/MAX262

Figure 21. Circuit for DC Offset Adjustment

TO

FILTER

INPUT

______________________________________________________________________________________ 23

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