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Original question:

[FAQ] DAC80501: Why is the output not updating?

DAC70501: DAC70501

Ernesto L
Prodigy 20 points
Part Number: DAC70501

I had the same issue as the one on this thread.

https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/994874/faq-dac80501-why-is-the-output-not-updating

And I did notice the REF-ALARM flag on the STATUS register so figured out what was the issue with that. The question came when I tried to make both the Gain and the DIV 1 so basically loading a 0000 on register GAIN. That didn't work. It does work if I load a 0101 on the GAIN register which basically makes GAIN = 2 and DIV = 2. Mathematically, both should cancel each other the same way but for some reason the result is not the same.

Is it the way that they are internally used? Hopefully somebody can help me understand this issue better,

  • 0
    TI__Mastermind 29718 points

    Hi Ernesto, 

    The REF-ALARM issue happens at the reference buffer (highlighted in the figure). 

    You're right that the two cases you mentioned would result in the same output range on the DAC output (from the output buffer). But when DIV=1 the reference buffer is seeing a value that is too large which causes it to shut down and trigger the reference alarm. 

    Best,

    Katlynne 

  • 0 in reply to Katlynne Jones
    Prodigy 20 points

    Thanks for your reply! As I suspected it sounds like is the way the internal logic uses those bits and in what order. It is still unclear to me why but I guess without a more detailed block diagram on the datasheet that will have to suffice. It is unfortunate that the datasheet doesn't include more information about that alarm. Given that the operating voltage range starts at 2.7V, I can see this being an issue often.

    Anyway, thanks again for your quick response.

  • 0 in reply to Ernesto L
    TI__Mastermind 29718 points

    Hi Ernesto,

    It's more about where the division/gain is being applied in the signal chain rather than the order of the internal logic. The reference buffer needs a sufficient amount of headroom to operate depending on the supply voltage used. This is why the reference divider is required to be used so the reference (internal or external) is divided down before going into the reference buffer. This has nothing to do with the output or the BUF-GAIN setting. If there's not enough headroom at this buffer, it will shut down. 

    The input to the output amplifier is the output of the DAC (the R-2R block in the block diagram), and this has a range of 0 to VREF/REF-DIV. So the BUF-GAIN bit allows you to add a 2x gain to this range which is useful if you needed to scale the reference down, or if you reference is smaller than VDD and you want the output range to be 0 to VDD.

    Hopefully that helps a little more. Let us know if you have any other questions.

    Best,

    Katlynne Jones 

  • 0 in reply to Katlynne Jones
    Prodigy 20 points

    Thanks, Katlynne! This information is certainly helpful and clarifies the issue for me. I still think the datasheet needs more detail around this alarm.