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AD9959 DAC output circuit: why data sheet and EVB schematic different?

Thread Summary

The user asked about discrepancies between the AD9959 DAC output circuit in the datasheet and the eval board schematic, specifically regarding the necessity of R7 and R8. The final answer confirmed that R7 and R8 (50Ω to AVDD) are essential for providing a DC load path to convert the DAC's current output to a voltage swing. The eval board includes additional components like transformers (T2–T5) and 50Ω resistors (R1–R4) for impedance matching and single-ended conversion to SMA outputs.
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Category: Hardware
Product Number: AD9959

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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  • Hi  ,

    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

Reply
  • Hi  ,

    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

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