AD9959 DAC output circuit in data sheet is different with EVB schematic. which one is correct? R7 and R8 are necessary? thanks


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AD9959
Recommended for New Designs
The AD9959 consists of four direct digital synthesizer (DDS)
cores that provide independent frequency, phase, and amplitude
control on each channel. This...
Datasheet
AD9959 on Analog.com
AD9959 DAC output circuit in data sheet is different with EVB schematic. which one is correct? R7 and R8 are necessary? thanks


Hi zhangmapdc ,
Apologies for responding to this late. I have been having problems with notifications.
R7 and R8 (50Ω to AVDD) are necessary. The AD9959 DAC outputs are current sources. These resistors provide the required DC load path to convert output current into a voltage swing. Without them, the DAC outputs have no defined DC operating point.
The datasheet shows the simplified/minimal application circuit (resistor load to AVDD, possibly direct into a load). The eval board adds transformers (T2–T5) and additional 50Ω resistors (R1–R4) on the primary side for impedance matching and single-ended conversion to the SMA outputs.
All the best,
Jules
Hi zhangmapdc ,
Apologies for responding to this late. I have been having problems with notifications.
R7 and R8 (50Ω to AVDD) are necessary. The AD9959 DAC outputs are current sources. These resistors provide the required DC load path to convert output current into a voltage swing. Without them, the DAC outputs have no defined DC operating point.
The datasheet shows the simplified/minimal application circuit (resistor load to AVDD, possibly direct into a load). The eval board adds transformers (T2–T5) and additional 50Ω resistors (R1–R4) on the primary side for impedance matching and single-ended conversion to the SMA outputs.
All the best,
Jules