AD6620AS datasheet(24/43 Pages) AD

24 / 43 page

AD6620

–24–

REV. 0

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 com-

pensated for in the RCF stage.

CIC

()

–

=×









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 dy-

namic range, SCIC5 should be set to the smallest value possible

(lowest attenuation) without creating an overflow condition.

This can be safely accomplished using the equation below,

where OLCIC2 is the largest fraction of full scale possible at the

input to this filter stage. This value is output from the CIC2

stage then pipe-lined into the CIC5. SCIC5 is ignored when this

filter is bypassed by setting MCIC5 = 1.

ceil

CIC

=×

















log (

) –

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

SAMP

CIC

≤

CIC5 Rejection

The table below illustrates the amount of bandwidth in percent-

age of the clock rate that can be protected with various decima-

tion rates and alias rejection specifications. The maximum input

rate into the CIC5 is 32.5 MHz. As in the previous table, these

are the 1/2 bandwidth characteristics of the CIC5. Note that the

CIC5 stage can protect a much wider band to any given rejection.

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

This table helps to calculate an upper bound on decimation,

MCIC5, given the desired filter characteristics.

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