AD9779PCB datasheet(26/34 Pages) AD

26 / 34 page

AD9779

Preliminary Technical Data

Rev. PrD | Page 26 of 34

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Figure 44. Interpolation/Modulation Combination of fDAC/8

Filter in Odd Mode

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Figure 45. Interpolation/Modulation Combination of 2fDAC/8

Filter in Odd Mode

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Figure 46. Interpolation/Modulation Combination of 3fDAC/8

Filter in Odd Mode

Even mode filter responses allow the passband to be centered

around ±0.5, ±1.5, ±2.5 and ±3.5 FDATA. Switching from and odd

mode response to an even mode filter response does not modulate

the signal. Instead, the pass band is simply shifted. As an example,

picture the response of Figure 46, and assume the signal in band is

a complex signal over the bandwidth 3.2 to 3.3×FDATA. If the even

mode filter response is then selected, the pass band will now be

centered at 3.5×FDATA. However, the signal will still remain at the

same place in the spectrum. The even/odd mode capability allows

the passband to be placed anywhere in the DAC Nyquist

bandwidth.

The AD9779 is a dual DAC with an internal complex modulator

built into the interpolating filter response. The modulator can be

set to a real or a complex mode by programming register 02h, bit 5.

In the default mode, bit 5 is set to zero and the modulation is

complex. The AD9779 then expects the real and the imaginary

components of a complex signal at digital input ports one and two

(I and Q respectively). The DAC outputs will then represent the

real and imaginary components of the input signal, modulated by

the complex carrier FDAC/2, FDAC/4 or FDAC/8.

With Bit 5 set to one, the modulation is real. The Q channel is shut

off and it’s value at the modulator inputs replaced with zero. The

output spectrum at either the IDAC or the QDAC will then

represent the signal at digital input port one, real modulated by the

internal digital carrier (FDAC/2, FDAC/4 or FDAC/8).

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