AD6620SSLASHPCB datasheet(24/44 Pages) AD

24 / 44 page

AD6620

–24–

REV. A

This table helps to calculate an upper bound on decimation,

MCIC5, given the desired filter characteristics.

The plots following (Figure 38) represent the CIC5 transfer

function with respect to the CIC5 output rate for a decimation

of 4. The first plot is referenced to the input sample rate and

shows the complex spectrum from –fSAMP/2 to +fSAMP/2. The

second plot is referenced to the CIC5 output rate; the complex

spectrum ranges from –fSAMP5/2 to +fSAMP5/2. Aliased images in

CIC5 “fold back” toward the edge of the desired filter pass

band. It is the level of these aliases as they move into the desired

pass band that are of concern. The improved roll-off of CIC5

over CIC2 can be seen when these plots are compared to those

previously shown for CIC2.

–0.5

–120

–100

–80

–60

–40

–20

f/fSAMP

–0.4 –0.3

–0.2

–0.1

0.1

0.2

0.3

0.4

0.5

–0.5

–120

–100

–80

–60

–40

–20

f/fSAMP5

–0.4 –0.3

–0.2

–0.1

0.1

0.2

0.3

0.4

0.5

Figure 38. CIC5 Alias Rejection, MCIC5 = 4

CIC5 Rejection

The table below illustrates the amount of bandwidth in percent-

age of the clock rate that can be protected with various decimation

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 than the CIC2 for

any given rejection.

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

Bandwidth Shown in Percentage of fSAMP2

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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