AD9776-EB datasheet(33/56 Pages) AD

33 / 56 page

AD9776/AD9778/AD9779

Rev. A | Page 33 of 56

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fOUT (× Input Data Rate)

Figure 65. Interpolation/Modulation Combination of fDAC/8 Filter

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Figure 66. Interpolation/Modulation Combination of

2 fDAC/8 Filter in Shifted Mode

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fOUT (× Input Data Rate)

Figure 67. Interpolation/Modulation Combination of

3 fDAC/8 Filter in Shifted Mode

Shifted mode filter responses allow the pass band to be centered

around ±0.5 fDATA, ±1.5 fDATA, ±2.5 fDATA, and ±3.5 fDATA. Switching

to the shifted mode response does not modulate the signal.

Instead, the pass band is simply shifted. For example, picture

the response shown in Figure 67 and assume the signal in-band

is a complex signal over the bandwidth 3.2 fDATA to 3.3 fDATA. If

the even mode filter response is then selected, the pass band

becomes centered at 3.5 fDATA. However, the signal 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 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 fDAC/2,

fDAC/4, or fDAC/8.

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 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 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×.

The general recommendation is that if the desired signal is

within ±0.4 × fDATA, the odd filter mode should be used. Outside

of this, the even filter mode should be used. In any situation, the

total bandwidth of the signal should be less than 0.8 × fDATA.

AD9776-EB Datasheet(PDF) 33 Page - Analog Devices

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