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DAC80508: Output Voltage Saturation

Part Number: DAC80508

Hello,

I have an application with the following requirements:

  1. Using the internal reference (which is 2.5V) with a divider value = 1
  1. The DAC output gain will be set to 2x 
  1. VDD will be 4.7V
  2. DAC FSR = 5V @ +/-15mA (max), though we will only drive it between 3.7V and 4.5V maximum.

Questions:

  1. What will happen if we set the DAC output to 5V with a 4.7V supply?   I suspect that the output will saturate at 4.7V - DAC DC Output Resistance x DAC output load current.    
  2. Will there be any potential for damage to the device by leaving it in this condition for a long or short time?
  3. Do we have any curves that detail the DAC output voltage droop given a specific load current and supply voltage?

Don

  • Don,

    Akhilesh is the primary support for us for this device, but as he is located in India I am jumping in briefly to give you a short-term response.

    The datasheet is perhaps not as clear as it could be on this matter, but there is some logic inside the device to compare the reference voltage to the supplies. If the device decides that the two are incompatible, it will throw a "reference error" and disable the internal reference buffer, which as a result will drive all of the DAC outputs to 0V. This is discussed in 8.3.2.1 on page 23.

    This section refers to the ROC table for valid configurations of reference and supply. Divider set to 1 and gain set to 2 with 3.3V to 5.5V supplies indicates a maximum VREFIN value of VDD / 2. So in the case with 4.7V and 2.5V reference, the error will be thrown and the internal buffer will be disabled.

    While I don't necessarily agree with the level of coercion, I do also understand that it would come across as odd to most designers or systems engineers to design around expecting saturation as, to some extent, it could be unpredictable.

    Of course, this response bears impact to your questions 2 and 3. I suppose I would suggest that you try to recommend a solution with 5V supply, referring to page 8 of the datasheet this would give you 0.5V of headroom vs your expected 4.5V maximum output, which gives you the maximum 20mA source/sink current capability test condition in the datasheet - which would align with the requirements you've mentioned for 15mA.

  • Thanks Kevin,

    I appreciate the detailed response.   That's unfortunate.   

    My application already has a 4.7V supply (which I'm using for another analog component), and I need control voltages up to ~4.5V.   In most cases, the DAC output voltage will be far less than 4.7V thus saturation would be avoided.   Given the reference shutdown operation, I will require a second supply in my already space constrained system.

    Is there a register option to disable/mask the reference check?

    Don

  • Don,

    No - at least not that is publicly documented. We can chat offline to go into more details. I'll send you an email.