• State TI Thinks Resolved
  • Locked Locked
  • Replies 8 replies
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 600 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DAC80502: Designing with DAC80502

Shibin Varghese
Intellectual 835 points
Part Number: DAC80502

Hello Team,
We are designing a 0-10V analog output using the DAC DAC80502DRXT.
The VREFIO is powered by the reference IC REF5040AID (4V).
The DAC is powered by a 5V supply (VDD=5V).
The DAC is followed by a non-inverting amplifier with a gain of 2V/V.
So the DAC voltage is doubled by the amplifier.
A 5V out from the DAC will give 10V out from the opamp.
The output of the DAC should swing within the range of 0-5V.
But when we set the configuration  BUFF-GAIN bit set to 1 and REF-DIV bit set to 0 the DAC output becomes. What may be the reason for this?

Attached is the circuit.


Also, can I set the DAC to internal reference if the external reference voltage is present at the VREFIO pin?
The external reference voltage we are using is 4V.

  • 0
    TI__Mastermind 42273 points

    Hi Shibin,

    The reference buffer requires a certain amount of headroom to operate. The buffer will shut down and the DAC outputs will go to 0 if the headroom requirement is not met. 

    For a 5-V supply and 4-V reference, the REF-DIV bit must be set to zero: 

     

    If you want a 0-5-V output, you would need to use the internal reference with BUF-GAIN and REF-DIV set to 1. Or use an external 5-V reference with BUF-GAIN and REF-DIV set to 0. You should not operate the DAC with both the internal and external reference enabled long term. 

    Another comment. Even if you were able to use the 4-V reference with BUF-GAIN set to 1 and REF-DIV set to 0, you would be losing almost half of the 16-bit resolution. The ideal full scale range would be 0-8V, but the DAC output buffer can only output 0-VDD. So, the 5V-8V of the ideal full scale range would be saturated to 5V, and you wouldn't see any change in output for codes greater than 5V.

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Intellectual 835 points

    Hello Katlynne,
    Thank you for your reply.
    Now we are using the internal voltage Ref of the DAC (2.5V) and we are getting proper results.
    Can you please explain why we are getting an error while configuring like BUFF-GAIN bit set to 1 and REF-DIV bit set to 0 with 4BUFF-GAIN bit set to 1 and REF-DIV bit set to 0 while setting the REF voltage to external 4V?


    Also, can use a 5V reference IC like REF5050 when the VDD of the DAC is at 5V


    Looking for your reply.

  • 0 in reply to Shibin Varghese
    TI__Mastermind 42273 points

    Hi Shibin,

    The reference buffer will shut down if it does not have enough headroom. This is true when using the internal reference or an external reference. When not divided (REF-DIV=0), your 4-V reference is too high and the reference buffer is shutting down. Please see the screenshots from the datasheet in my last response. 

    Best,

    Katlynne 

  • 0 in reply to Katlynne Jones
    TI__Mastermind 42273 points

    One correction to my last response, you could use the 2.5-V internal reference with BUF-GAIN set to 1 and REF-DIV set to 0, an external 2.5-V reference with BUF-GAIN set to 1 and REF-DIV set to 0, or an external 5-V reference (like the REF5050) with BUF-GAIN set to 1 and REF-DIV set to 1. All cases will still give you a 0-5V output. 

  • 0 in reply to Katlynne Jones
    Intellectual 835 points

    Hello Katlynne,
    Thank you for your reply.
    We have updated the circuit as per your suggestions.
    Now the REF voltage is set to 5V.
    The VDD of the DAC is 5V.
    We need an output voltage of 0V to 5V.



    Looking for your reply.

  • 0 in reply to Shibin Varghese
    TI__Mastermind 42273 points

    Hi Shibin, 

    This looks fine. Set BUF-GAIN to 1, and REF-DIV to 1 to get the 0-5V output. You must set REF-DIV to 1, otherwise the reference buffer will shut down and you will not see any output. 

    The internal reference is enabled by default, make sure to disable the internal reference after powering on the device. You can also add a series current limiting resistor to limit the current into the pin during the time that the internal and external reference are both on. 

    Best,

    Katlynne Jones 

  • 0 in reply to Katlynne Jones
    Intellectual 835 points

    Hello Katlynne,
    Thank you for your reply.
    Just to make things clear.
    The DAC is powered from a 5V at its VDD pin.
    To get a 0 -5V output from DAC,

    REF5050AID can be used with
    BUF-GAIN Set to 1
    REF-DIV Set to 1

    REF5025AID can also be used with
    BUF-GAIN Set to 1
    REF-DIV Set to 0

     Please correct me if I am wrong

  • 0 in reply to Shibin Varghese
    TI__Mastermind 42273 points

    Hi Shibin,

    Yes, that is correct.

    Or the internal reference can be used with 

    BUF-GAIN Set to 1
    REF-DIV Set to 0

    Best,

    Katlynne Jones