AD9914SLASHPCBZ datasheet(21/48 Pages) AD

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Data Sheet

AD9914

FUNCTIONAL BLOCK DETAIL

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Rev. G | 21 of 48

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 estab-

lishes 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:

tCAL=469,632fS

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 32) 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 out-

put, 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

followed by a transition band where the response rolls off as steeply

as possible, and then a stop band that 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 fre-

quency, 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.

Figure 32. DAC Spectrum vs. Reconstruction Filter Response

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