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AD9914

Data Sheet

Rev. F | Page 20 of 45

12-BIT DAC OUTPUT

The AD9914 incorporates an integrated 12-bit, current output

DAC. The output current is delivered as a balanced signal using

two outputs. The use of balanced outputs reduces the potential

amount of common-mode noise present at the DAC output,

offering the advantage of an increased signal-to-noise ratio. An

external resistor (RSET) connected between the DAC_RSET pin

and AGND establishes the reference current. The recommended

value of RSET is 3.3 kΩ.

Attention must be paid to the load termination to keep the

output voltage within the specified compliance range; voltages

developed beyond this range cause excessive distortion and can

damage the DAC output circuitry.

DAC CALIBRATION OUTPUT

The DAC CAL enable bit in the CFR4 control register (0x03[24])

must be manually set and then cleared after each power-up and

every time the REF CLK or internal system clock is changed.

This initiates an internal calibration routine to optimize the

setup and hold times for internal DAC timing. Failure to

calibrate may degrade performance and even result in loss of

functionality. The length of time to calibrate the DAC clock is

calculated from the following equation:

CAL

632

469

RECONSTRUCTION FILTER

The DAC output signal appears as a sinusoid sampled at fS. The

frequency of the sinusoid is determined by the frequency tuning

word (FTW) that appears at the input to the DDS. The DAC

output is typically passed through an external reconstruction

filter that serves to remove the artifacts of the sampling process

and other spurs outside the filter bandwidth.

Because the DAC constitutes a sampled system, the 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 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 numbered images (shown in Figure 31) 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 must 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 from 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.

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 31. DAC Spectrum vs. Reconstruction Filter Response

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