AD9857AST datasheet(24/41 Pages) AD

24 / 41 page

AD9857

Rev. C| Page 23 of 40

The output compliance voltage of the AD9857 is −0.5 V to

+1.0 V. Any signal developed at the DAC output should not

exceed 1.0 V, otherwise, signal distortion results. Furthermore,

the signal may extend below ground as much as 0.5 V without

damage or signal distortion. The use of a transformer with a

grounded center tap for common-mode rejection results in

signals at the AD9857 DAC output pins that are symmetrical

about ground.

As previously mentioned, by differentially combining the two

signals, the user can provide some degree of common-mode

signal rejection. A differential combiner might consist of a

transformer or an op amp. The object is to combine or amplify

only the difference between two signals and to reject any

common, usually undesirable, characteristic, such as 60 Hz

hum or clock feed-through that is equally present on both input

signals. The AD9857 true and complement outputs can be

differentially combined using a broadband 1:1 transformer with

a grounded, center-tapped primary to perform differential

combining of the two DAC outputs.

REFERENCE CLOCK MULTIPLIER

It is often difficult to provide a high quality oscillator with an

output in the frequency range of 100 MHz – 200 MHz. The

AD9857 allows the use of a lower-frequency oscillator that can

be multiplied to a higher frequency by the on-board reference

clock multiplier, implemented with a phase locked loop

architecture. See the Ease of Use Features section for a more

thorough discussion of the reference clock multiplier feature.

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