AD6636BBCZ1 datasheet(29/72 Pages) AD

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AD6636

Rev. 0 | Page 29 of 72

Table 13 can be used to decide the minimum decimation

required in the CIC stage to preserve a certain bandwidth. The

CIC5 stage can protect a much wider bandwidth to any given

rejection, when a decimation ratio lower than that identified in

the table is used. The table helps to calculate an upper boundary

on decimation, MCIC, given the desired filter characteristics.

Table 13. SSB CIC5 Alias Rejection Table (fin = 1)

MCIC5

−60 dB

−70 dB

−80 dB

−90 dB

−100 dB

8.078

6.393

5.066

4.008

3.183

6.367

5.11

4.107

3.297

2.642

5.022

4.057

3.271

2.636

2.121

4.107

3.326

2.687

2.17

1.748

3.463

2.808

2.27

1.836

1.48

2.989

2.425

1.962

1.588

1.281

2.627

2.133

1.726

1.397

1.128

2.342

1.902

1.54

1.247

1.007

2.113

1.716

1.39

1.125

0.909

1.924

1.563

1.266

1.025

0.828

1.765

1.435

1.162

0.941

0.76

1.631

1.326

1.074

0.87

0.703

1.516

1.232

0.998

0.809

0.653

1.416

1.151

0.932

0.755

0.61

1.328

1.079

0.874

0.708

0.572

1.25

1.016

0.823

0.667

0.539

1.181

0.96

0.778

0.63

0.509

1.119

0.91

0.737

0.597

0.483

1.064

0.865

0.701

0.568

0.459

1.013

0.824

0.667

0.541

0.437

0.967

0.786

0.637

0.516

0.417

0.925

0.752

0.61

0.494

0.399

0.887

0.721

0.584

0.474

0.383

0.852

0.692

0.561

0.455

0.367

0.819

0.666

0.54

0.437

0.353

0.789

0.641

0.52

0.421

0.34

0.761

0.618

0.501

0.406

0.328

0.734

0.597

0.484

0.392

0.317

0.71

0.577

0.468

0.379

0.306

0.687

0.559

0.453

0.367

0.297

0.666

0.541

0.439

0.355

0.287

Example Calculations

Goal:

Implement a filter with an input sample rate of 100 MHz

requiring 100 dB of alias rejection for a ± 1.4 MHz pass band.

Solution:

First determine the percentage of the sample rate that

is represented by the pass band.

MHz

100

MHz

100

fraction

In the −100 dB column in Table 13, find the value greater than

or equal to the pass-band percentage of the clock rate. Then

find the corresponding rate decimation factor (MCIC). For an

MCIC of 6, the frequency that has −100 dB of alias rejection is

1.48%, which is slightly larger than the 1.4% calculated.

Therefore, for this example, the maximum bound on CIC

decimation rate is 6. A higher MCIC means less alias rejection

than the 100 dB required.

FIR HALF-BAND BLOCK

The output of the CIC filter is pipelined into the FIR HB (half-

band) block. Each channel has two sets of cascading fixed-

coefficient FIR and fixed-coefficient half-band filters. The half-

band filters decimate by 2. Each of these filters (FIR1, HB1,

FIR2, HB2) are described in the following sections.

3-Tap Fixed-Coefficient Filter (FIR1)

The 3-tap FIR filter is useful in certain filter configurations in

which extra alias protection is needed for the decimating HB1

filter. It is a simple sum-of-products FIR filter with three filter

taps and 2-bit fixed coefficients. Note that this filter does not

decimate. The coefficients of this symmetric filter are {1, 2, 1}.

The normalized coefficients used in the implementation are

{0.25, 0.5, 0.25}.

The user can either use or bypass this filter. Writing Logic 0 to

the FIR1 enable bit in the FIR-HB control register bypasses this

fixed-coefficient filter. The filter is useful only in certain filter

configurations and bypassing it for other applications results in

power savings.

FRACTION OF FIR1 INPUT SAMPLE RATE

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

–16.67

–8.33

–33.33

–25.00

–50.00

–41.67

–66.67

–58.33

–83.33

–75.00

–100.00

–91.67

0.34

0.66

–81

FIR1 RESPONSE

Figure 31. FIR1 Filter Response to the Input Rate of the Filter

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