AD9912ASLASHPCBZ datasheet(19/40 Pages) AD

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AD9912

Rev. F | Page 19 of 40

PRIMARY

SIGNAL

FILTER

RESPONSE

SIN(x)/x

ENVELOPE

SPURS

IMAGE 0

IMAGE 1

IMAGE 2

IMAGE 3

IMAGE 4

–20

–40

–60

–80

–100

MAGNITUDE

(dB)

fs/2

BASE BAND

Figure 42. DAC Spectrum vs. Reconstruction Filter Response

Because the DAC constitutes a sampled system, its output must

be filtered so that the analog waveform accurately represents the

digital samples supplied to the DAC input. The unfiltered DAC

output contains the (typically) desired baseband signal, which

extends from dc to the Nyquist frequency (fS/2). It also contains

images of the baseband signal that theoretically extend to infinity.

Notice that the odd images (shown in Figure 42) are mirror

images of the baseband signal. Furthermore, the entire DAC

output spectrum is affected by a sin(x)/x response, which is

caused by the sample-and-hold nature of the DAC output signal.

For applications using the fundamental frequency of the DAC

output, the response of the reconstruction filter should preserve

the baseband signal (Image 0), while completely rejecting all

other images. However, a practical filter implementation

typically exhibits a relatively flat pass band that covers the

desired output frequency plus 20%, rolls off as steeply as

possible, and then maintains significant (though not complete)

rejection of the remaining images. Depending on how close

unwanted spurs are to the desired signal, a third-, fifth-, or

seventh-order elliptic low-pass filter is common.

Some applications operate off an image above the Nyquist

frequency, and those applications use a band-pass filter instead

of a low-pass filter.

The design of the reconstruction filter has a significant impact

on the overall signal performance. Therefore, good filter design

and implementation techniques are important for obtaining the

best possible jitter results.

FDBK_IN INPUTS

The FDBK_IN pins serve as the input to the comparators and

output drivers of the AD9912. Typically, these pins are used to

receive the signal generated by the DDS after it has been band-

limited by the external reconstruction filter.

A diagram of the FDBK_IN input pins is provided in Figure 43,

which includes some of the internal components used to bias

the input circuitry. Note that the FDBK_IN input pins are

internally biased to a dc level of ~1 V. Care should be taken to

ensure that any external connections do not disturb the dc bias

because this may significantly degrade performance.

15k

Ω

15k

Ω

~1pF

TO S-DIVIDER

AND CLOCK

OUTPUT SECTION

AVSS

~1V

AVSS

~2pF

FDBK_IN

FDBK_INB

Figure 43. Differential FDBK_IN Inputs

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