AD9856SLASHPCB datasheet(25/37 Pages) AD

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AD9856

Rev. C | Page 24 of 36

Table 6. CIC Interpolation Filter Insertion Loss Table

Default Gain

2 × Gain

Interpolation

Rate

(Linear)

(dB)

(Linear)

(dB)

1.0000

0.000

2.0000

6.021

0.8438

−1.476

1.6875

4.545

1.0000

0.000

2.0000

6.021

0.9766

−0.206

1.9531

5.815

0.8438

−1.476

1.6875

4.545

0.6699

−3.480

1.3398

2.541

1.0000

0.000

2.0000

6.021

0.7119

−2.951

1.4238

3.069

0.9766

−0.206

1.9531

5.815

0.6499

−3.743

1.2998

2.278

0.8438

−1.476

1.6875

4.545

0.5364

−5.411

1.0728

0.610

0.6699

−3.480

1.3398

2.541

0.8240

−1.682

1.6479

4.339

1.0000

0.000

2.0000

6.021

0.5997

−4.441

1.1995

1.580

0.7119

−2.951

1.4238

3.069

0.8373

−1.543

1.6746

4.478

0.9766

−0.206

1.9531

5.815

0.5652

−4.955

1.1305

1.065

0.6499

−3.743

1.2998

2.278

0.7426

−2.585

1.4852

3.436

0.8438

−1.476

1.6875

4.545

0.9537

−0.412

1.9073

5.609

0.5364

−5.411

1.0728

0.610

0.6007

−4.427

1.2014

1.593

0.6699

−3.480

1.3398

2.541

0.7443

−2.565

1.4886

3.455

0.8240

−1.682

1.6479

4.339

0.9091

−0.827

1.8183

5.193

1.0000

0.000

2.0000

6.021

0.5484

−5.219

1.0967

0.802

0.5997

−4.441

1.1995

1.580

0.6542

−3.686

1.3084

2.335

0.7119

−2.951

1.4238

3.069

0.7729

−2.237

1.5458

3.783

0.8373

−1.543

1.6746

4.478

0.9051

−0.866

1.8103

5.155

0.9766

−0.206

1.9531

5.815

0.5258

−5.583

1.0517

0.437

0.5652

−4.955

1.1305

1.065

0.6066

−4.342

1.2132

1.679

0.6499

−3.743

1.2998

2.278

0.6952

−3.157

1.3905

2.863

0.7426

−2.585

1.4852

3.436

0.7921

−2.024

1.5842

3.996

0.8438

−1.476

1.6875

4.545

0.8976

−0.938

1.7952

5.082

Default Gain

2 × Gain

Interpolation

Rate

(Linear)

(dB)

(Linear)

(dB)

0.9537

−0.412

1.9073

5.609

0.5060

−5.917

1.0120

0.104

0.5364

−5.411

1.0728

0.610

0.5679

−4.914

1.1358

1.106

0.6007

−4.427

1.2014

1.593

0.6347

−3.949

1.2693

2.072

0.6699

−3.480

1.3398

2.541

0.7065

−3.018

1.4129

3.002

0.7443

−2.565

1.4886

3.455

0.7835

−2.120

1.5669

3.901

0.8240

−1.682

1.6479

4.339

0.8659

−1.251

1.7317

4.770

0.9091

−0.827

1.8183

5.193

0.9539

−0.410

1.9077

5.610

INVERSE SINC FILTER (ISF)

The AD9856 is almost entirely a digital device. The input signal

is made up of a time series of digital data-words. These data-

words propagate through the device as numbers. Ultimately,

this number stream must be converted to an analog signal.

To this end, the AD9856 incorporates an integrated DAC.

The output waveform of the DAC is the familiar staircase

pattern typical of a signal that is sampled and quantized. The

staircase pattern is a result of the finite time that the DAC holds

a quantized level until the next sampling instant. This is known

as a zero-order hold function. The spectrum of the zero-order

hold function is the SIN(x)/x, or SINC, envelope.

The series of digital data-words presented at the input of the

DAC represent an impulse stream. It is the spectrum of this

impulse stream, which is the desired output signal. Due to

the zero-order hold effect of the DAC, however, the output

spectrum is the product of the zero-order hold spectrum

(the SINC envelope) and the Fourier transform of the impulse

stream. Thus, there is an intrinsic distortion in the output

spectrum, which follows the SINC response.

The SINC response is deterministic and totally predictable.

Thus, it is possible to predistort the input data stream in a

manner that compensates for the SINC envelope distortion.

This can be accomplished by means of an ISF. The ISF

incorporated on the AD9856 is a 17-tap, linear phase FIR filter.

Its frequency response characteristic is the inverse of the SINC

envelope. Data sent through the ISF is altered to correct for the

SINC envelope distortion.

Note, however, that the ISF is sampled at the same rate as

the DAC. Thus, the effective range of the SINC envelope

compensation only extends to the Nyquist frequency (1/2 of

the DAC sample rate).

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