Changes To Interpolation Filter Architecture Section - Analog Devices AD9776A Manual

10
0
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–90
–100
–4 –3
Figure 64. Interpolation/Modulation Combination of −f
10
0
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–4 –3
Figure 65. Interpolation/Modulation Combination of f
10
0
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Figure 66. Interpolation/Modulation Combination of 2f
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–2 –1 0 1 2 3
f
(× Input Data Rate)
OUT
DAC
–2 –1 0 1 2 3
f
(× Input Data Rate)
OUT
DAC
–2 –1 0 1 2 3
f
(× Input Data Rate)
OUT
DAC
10
0
–10
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–40
–50
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–90
–100
4 –4
/8 Filter Figure 67. Interpolation/Modulation Combination of 3f
Shifted mode filter responses allow the pass band to be centered
around ±0.5 f
the shifted mode response does not affect the center frequency
of the signal. Instead, the pass band of the filter is simply shifted.
For example, use the response shown in Figure 67 and assume
the signal in-band is a complex signal over the bandwidth 3.2 f
to 3.3 f
DATA
the pass band becomes centered at 3.5 f
remains at the same place in the spectrum. The shifted mode
capability allows the filter pass band to be placed anywhere in
the DAC Nyquist bandwidth.
The AD9776A/AD9778A/AD9779A are dual DACs with
internal complex modulators built into the interpolating filter
response. In dual channel mode, the devices expect the real and
4
the imaginary components of a complex signal at Digital Input
Port 1 and Digital Input Port 2 (I and Q, respectively). The DAC
/8 Filter
outputs then represent the real and imaginary components of
the input signal, modulated by the complex carrier (f
f /4, or f
DAC
With Register 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 is 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 Register 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×.
4
The general recommendation is that if the desired signal is
within ±0.4 × f
/8 Filter
this, the shifted filter mode should be used. In any situation, the
total bandwidth of the signal should be less than 0.8 × f
Rev. A | Page 33 of 60
AD9776A/AD9778A/AD9779A
–3 –2 –1 0 1
f
(× Input Data Rate)
OUT
, ±1.5 f , ±2.5 f , and ±3.5 f
DATA DATA DATA
. If the shifted mode filter response is then selected,
DATA
/8).
DAC
, use the nonshifted filter mode. Outside of
DATA
2 3 4
/8 Filter
DAC
. Switching to
DATA
DATA
. However, the signal
/2,
DAC
.
DATA