AD6634BCSLASHPCB datasheet(26/52 Pages) AD

26 / 52 page

REV. 0

–26–

AD6634

SAMP

CLK

2 ≤

The decimation ratio, MCIC5, may be programmed from 2 to 32

(all integer values). The frequency response of the filter is given

by the following equations. The gain and pass-band droop of

CIC5 should be calculated by these equations. Both parameters

may be compensated for in the RCF stage.

CIC

SAMP

CIC

() =

−









() =





























−

sin

The scale factor, SCIC5 is a programmable unsigned integer between

0 and 20. It serves to control the attenuation of the data into the

CIC5 stage in 6 dB increments. For the best dynamic range, SCIC5

should be set to the smallest value possible (lowest attenuation)

without creating an overflow condition. This can be safely accom-

plished using the following equation, where OLrCIC2 is the largest

fraction of full scale possible at the input to this filter stage. This

value is output from the rCIC2 stage then pipelined into the CIC5.

ceil

CIC

rCIC

CIC

rCIC

CIC

=×

()









= () ×

log

–

The output rate of this stage is given by the equation below.

SAMP

CIC

CIC5 Rejection

Table IV illustrates the amount of bandwidth in percentage of

the clock rate that can be protected with various decimation

rates and alias rejection specifications. The maximum input rate

into the CIC5 is 80 MHz when the rCIC2 decimates by 1. As in

the Table III, these are the one-half bandwidth characteristics of

the CIC5. Notice that the CIC5 stage can protect a much wider

band to any given rejection.

This table helps to calculate an upper bound on decimation,

MCIC5, given the desired filter characteristics.

Table IV. SSB CIC5 Alias Rejection Table (fSAMP2 = 1)

MCIC5

–50 dB

–60 dB

–70 dB

–80 dB

–90 dB

–100 dB

10.227

8.078

6.393

5.066

4.008

3.183

7.924

6.367

5.11

4.107

3.297

2.642

6.213

5.022

4.057

3.271

2.636

2.121

5.068

4.107

3.326

2.687

2.17

1.748

4.267

3.463

2.808

2.27

1.836

1.48

3.68

2.989

2.425

1.962

1.588

1.281

3.233

2.627

2.133

1.726

1.397

1.128

2.881

2.342

1.902

1.54

1.247

1.007

2.598

2.113

1.716

1.39

1.125

0.909

2.365

1.924

1.563

1.266

1.025

0.828

2.17

1.765

1.435

1.162

0.941

0.76

2.005

1.631

1.326

1.074

0.87

0.703

1.863

1.516

1.232

0.998

0.809

0.653

1.74

1.416

1.151

0.932

0.755

0.61

1.632

1.328

1.079

0.874

0.708

0.572

1.536

1.25

1.016

0.823

0.667

0.539

1.451

1.181

0.96

0.778

0.63

0.509

1.375

1.119

0.91

0.737

0.597

0.483

1.307

1.064

0.865

0.701

0.568

0.459

1.245

1.013

0.824

0.667

0.541

0.437

1.188

0.967

0.786

0.637

0.516

0.417

1.137

0.925

0.752

0.61

0.494

0.399

1.09

0.887

0.721

0.584

0.474

0.383

1.046

0.852

0.692

0.561

0.455

0.367

1.006

0.819

0.666

0.54

0.437

0.353

0.969

0.789

0.641

0.52

0.421

0.34

0.934

0.761

0.618

0.501

0.406

0.328

0.902

0.734

0.597

0.484

0.392

0.317

0.872

0.71

0.577

0.468

0.379

0.306

0.844

0.687

0.559

0.453

0.367

0.297

0.818

0.666

0.541

0.439

0.355

0.287

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