AD9779
Preliminary Technical Data
Rev. PrD | Page 26 of 34
-4
-3
-2
-1
0
1
2
3
4
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
Figure 44. Interpolation/Modulation Combination of f
DAC
/8
Filter in Odd Mode
-4
-3
-2
-1
0
1
2
3
4
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
Figure 45. Interpolation/Modulation Combination of 2f
DAC
/8
Filter in Odd Mode
-4
-3
-2
-1
0
1
2
3
4
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
Figure 46. Interpolation/Modulation Combination of 3f
DAC
/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 F
DATA
. 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
×
F
DATA
. If the even
mode filter response is then selected, the pass band will now be
centered at 3.5
×
F
DATA
. 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 F
DAC
/2, F
DAC
/4 or F
DAC
/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 (F
DAC
/2, F
DAC
/4 or F
DAC
/8).
