Analog Devices AD9776 Instruction Manual page 31

10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–4 –3 –2 –1
f
(× Input Data Rate)
OUT
Figure 62. Interpolation/Modulation Combination of f
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–4 –3 –2 –1
f
(× Input Data Rate)
OUT
Figure 63. Interpolation/Modulation Combination of
2f /8 Filter in Odd Mode
DAC
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–4 –3 –2 –1
f
(× Input Data Rate)
OUT
Figure 64. Interpolation/Modulation Combination of
3f /8 Filter in Odd Mode
DAC
0 1 2 3 4
/8 Filter
DAC
0 1 2 3 4
0 1 2 3 4
Rev. 0 | Page 31 of 56
AD9776/AD9778/AD9779
Shifted mode filter responses allow the pass band to be centered
around ±0.5, ±1.5, ±2.5, and ±3.5 f
mode response does not modulate the signal. Instead, the pass
band is simply shifted. For example, picture the response shown
in Figure 64 and assume the signal in-band is a complex signal
over the bandwidth 3.2 f
DATA
response is then selected, the pass band becomes centered at
3.5 f . However, the signal remains at the same place in the
DATA
spectrum. The shifted mode capability allows the filter pass
band to be placed anywhere in the DAC Nyquist bandwidth.
The AD9776/AD9778/AD9779 are dual DACs with internal
complex modulators built into the interpolating filter response.
In dual channel mode, the devices expect the real and the
imaginary components of a complex signal at Digital Input
Port 1 and Digital Input Port 2 (I and Q respectively). The DAC
outputs then represent the real and imaginary components of
the input signal, modulated by the complex carrier f
f /4, or f /8.
DAC DAC
With Reg. 2, Bit 6 set, the device accepts interleaved data on
Port 1 in the I, Q, I, Q ... sequence. Note that in interleaved
mode, the channel data rate at the beginning of the I and the Q
data paths are now half the input data rate because of the inter-
leaving. The maximum input data rate is still subject to the
maximum specification of the device. This limits the synthesis
bandwidth available at the input in interleaved mode.
With Reg. 0x02, Bit 5 (real mode) set, the Q channel and the
internal I and Q digital modulation are turned off. The output
spectrum at the I DAC then represents the signal at Digital
Input Port 1, interpolated by 1×, 2×, 4×, or 8×.
The general recommendation is that if the desired signal is
within ±0.4 × f , the odd filter mode should be used. Outside
DATA
of this, the even filter mode should be used. In any situation, the
total bandwidth of the signal should be less than 0.8 × f
. Switching to the shifted
DATA
to 3.3 f . If the even mode filter
DATA
/2,
DAC
DATA
.