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DAC7718: Calculation for output impedance

ACGUY
Guru 11170 points
Part Number: DAC7718

Hello E2E,

Please let me know how to calculate for output impedance of DAC7718.
I found it that 0.5-Ω when code = 0x from datasheet.
I want to know output impedance when from 0x000h to 0xFFFh.

Best regards,
ACGUY

  • 0
    TI__Mastermind 46290 points
    ACGUY,

    Can I ask exactly what you are hoping to use the output impedance figure for?

    For a DAC this is not necessarily straight forward, I've seen some pretty interesting DAC Zo frequency response curves in the past and there is some smaller dependency on code.

    Maybe if we understand what the goal is we can more directly address the concern.
  • 0 in reply to Kevin Duke
    Guru 11170 points

    Hello Kevin,

    Thank you for reply.

    >> Can I ask exactly what you are hoping to use the output impedance figure for?
    Are you requesting detail informartion about application or business volume?
    I can't post it in E2E.

    >> I've seen some pretty interesting DAC Zo frequency response curves in the past and there is some smaller dependency on code.
    What is the dependency of DAC's Zo?
    Is Zo a calculable parameter?

    Regards,
    ACGUY

  • 0 in reply to ACGUY
    TI__Mastermind 46290 points

    ACGUY,

    Sorry for the delays, I've been out of office on business travel for the last several days.

    ACGUY said:
    >> Can I ask exactly what you are hoping to use the output impedance figure for?
    Are you requesting detail informartion about application or business volume?
    I can't post it in E2E.

    I'm not necessarily asking for the application and not asking anything for the business volume. To phrase it directly, why is it necessary in this case to know more detail about the output impedance?

    ACGUY said:
    >> I've seen some pretty interesting DAC Zo frequency response curves in the past and there is some smaller dependency on code.
    What is the dependency of DAC's Zo?
    Is Zo a calculable parameter?

    Anything that is modeled via SPICE can be calculated, but it becomes more of a question of practicality. This is something you'd probably be better off with a SPICE model for than an equation for. 

  • 0 in reply to Kevin Duke
    TI__Mastermind 46290 points
    ACGUY,

    Any update concerning how we can go about helping you on this?
  • 0 in reply to Kevin Duke
    Guru 11170 points
    Kevin,

    I understood that output impedance does not to dependence for code by your comment.

    By the way, in the case of code 0x800, what does it mean that the output impedance is 0.5 - Ω?
    By that description I thought that Zo depended on Code.

    If I requested, would you measurement the Zo when codes 0x000 and 0xFFF?

    Regards,
    ACGUY
  • 0 in reply to ACGUY
    TI__Mastermind 46290 points

    ACGUY,

    I see - I finally understand the root cause of this question! Thanks.

    The reason the specification is provided at code 0x800 is that this is the mid-scale code of the DAC which is a fairly typical method of specifying DAC output impedance. The value to choosing the mid-scale code is output impedance will look different when the output amplifier is operating near either the positive or negative rails.

    So in the end when I said that there is no real dependency on code, perhaps I should have more carefully and precisely explained that within the linear region of operation there is very little, if any, dependency on code. Near the rails where the amplifier is non-linear, there would be some change in output impedance, which one might call a "code dependent" output impedance, though it's more directly the amplifier operating near a rail than it is anything intrinsic to the string DAC.

  • 0 in reply to Kevin Duke
    Guru 11170 points
    Many thanks!!