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DAC60508: Vout max with Internal Vref /1 and x2 Gain with VDD<5V

Part Number: DAC60508

I only have VDD=3.3V and need to generate Vout=2.6V so it would make sense to set the Vref /1 and Gain x2 to give me a Vout range of "0 V to 2 × VREF" as listed in Table 1.  Naturally I cannot achieve "2xVREF" (5.0 V) when VDD = 3.3V however the data sheet does not list this somewhat obvious limitation (or am I not reading the data sheet correctly?).  I would assume that Vout max then is VDD or maybe VDD-0.2 but assumptions will get me into trouble.  Please confirm that my assumption is correct and what is the Vout max for my situation.

Load requirements are < 1 mA

  • Barry,

    The device is intelligent enough to actually look at the reference source and leverage internal comparators to make a decision about what it can and cannot do. So if you use a 1.3V external reference with 3.3V VDD VREF / 1 and GAIN = 2, so you would get what you're interested in since you still meet the headroom requirements as you're using the external reference.

  • Duke, Thanks for the quick reply.  However, I am intending on using the internal reference, 2.5V.  Does this change your answer?

    Looking again at the data sheet, I see the spec "Output voltage headroom" which implies that with my load requirements less than 1 mA, then I can expect that I can achieve an output up to 3.15V with VDD=3.3V and the internal reference  with settings VREF / 1 and GAIN = 2.  I only need to achieve 2.6V which gives me more than enough headroom.  The definition of "output voltage headroom" doesn't state this is with respect to VDD but this is the only logical conclusion.

    Regards, Barry

  • Barry,

    I do believe the device is able to make dynamic decisions, so this is not just at power-up, but I am not sure how savvy it is to load current-consumption factors or if that commentary is in place in the document as more of an informational specification to prevent issues during normal operation.

    I do not have a board on hand myself, but someone on my team should be able to confirm this for you by tomorrow.

  • Hi Barry,

    Thanks for reaching out. The main limitation is on VREF as specified in section 7.3 of the datasheet. When you are using a VDD of 3.3V and the internal reference, the only option to get the output is by selecting VREF/2 using the REFDIV bit. So, with GAIN=2, you can get the full-scale output of 2.5V. The output voltage headroom doesn't come in to picture here. So, you cannot reach 2.6V in this configuration, unfortunately. This is a certain tradeoff in the design to achieve the specified accuracy. If you can manage with 2.5V output range, you can directly go ahead with the design.

    To get an output beyond 2.5V, there are three options:

    1. Increase the VDD to 5V so that the 2.5V internal reference can be used without dividing by 2

    2. Use a suitable external reference less than (3.3V - 0.2V)/2

    3. Gain up the output using an external amplifier

    Hope that answers your question. Let me know if you need further support.

    Regards,

    Uttam Sahu

    Applications Engineer, Precision DAC

  • Hi Uttam,

    Thanks for the details and my conclusion is that option 2 is the best solution for my case since I do not have available 5V and not enough space for 8 buffers.

    I trust your knowledge on the part however I would like to understand how you know this particular detail from the data sheet: "When you are using a VDD of 3.3V and the internal reference, the only option to get the output is by selecting VREF/2 using the REFDIV bit"

    Best Regards, Barry