AD9540 datasheet(20/32 Pages) AD

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AD9540

Data Sheet

Rev. C | Page 20 of 32

DDS AND DAC

The precision frequency division within the device is

accomplished using DDS technology. The DDS can control the

digital phase relationships by clocking a 48-bit accumulator.

The incremental value loaded into the accumulator, known as

the frequency tuning word, controls the overflow rate of the

accumulator. Similar to a sine wave completing a 2π radian

revolution, the overflow of the accumulator is cyclical in nature

and generates a fundamental frequency according to

)

( s

FTW

}

≤

≤ FT

The instantaneous phase of the sine wave is therefore the output

of the phase accumulator block. This signal can be phase-offset

by programming an additive digital phase that is added to each

phase sample coming out of the accumulator.

These instantaneous phase values are then piped through a

phase-to-amplitude conversion (sometimes called an angle-to-

amplitude conversion or AAC) block. This algorithm follows a

COS(x) relationship, where x is the phase coming out of the

phase offset block, normalized to 2π.

Finally, the amplitude words are piped to a 10-bit DAC. Because

the DAC is a sampled data system, the output is a reconstructed

sine wave that needs to be filtered to take high frequency images

out of the spectrum. The DAC is a current steering DAC that is

AVDD referenced. To get a measurable voltage output, the DAC

outputs must be terminated through a load resistor to AVDD,

typically 50 Ω. At positive full scale, IOUT sinks no current and

the voltage drop across the load resistor is 0. However, the IOUT

output sinks the programmed full-scale output current of the

DAC, causing the maximum output voltage drop across the load

resistor. At negative full-scale, the situation is reversed and IOUT

sinks the full-scale current (and generates the maximum drop

across the load resistor), while IOUT sinks no current (and

generates no voltage drop). At midscale, the outputs sink equal

amounts of current, generating equal voltage drops.

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