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DAC7811: Vref generation not working

Vasudeo Bidve
Prodigy 20 points
Part Number: DAC7811

Hi there.

I am trying to build the Vref generation circuit as given on the datasheet of DAC7811 fig. 29 Positive voltage output circuit. I am using a +2.5V ref generator IC from Analog devices - only because it was ready there in my stock. But I am not getting -2.5V at the Vref pin with respect to the physical ground. I am getting 0V there. What am I missing here?

Thanks and Regards

Vasudeo

  • 0
    TI__Mastermind 28058 points

    Hi Vasudeo,

    If you have verified that all of the connections have been made properly, it's possible that this level shifting scheme won't work with the reference you are using. Can you share the part number?

    Best,

    Katlynne

  • 0 in reply to Katlynne Jones
    Prodigy 20 points

    Hi Katlynne,

    Yes the connections are all made properly and confirmed multiple times. Here are the part numbers:

    DAC: DAC7811

    OpAmp: MCP6476

    Reference generator: ADR4525

    I used non-TI parts here only because they were there ready in our stock. Hope, that wouldn't make much difference.

  • 0 in reply to Vasudeo Bidve
    TI__Mastermind 28058 points

    Hi Vasudeo,

    Your TI account manager sent me your schematic via email. You have connected the negative supply of the op amp to GND. The output of the op amp cannot be more negative that it's supply which is why you are not seeing the expected output. 

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Prodigy 20 points

    Hi Katlynne,

    Thank you for pointing out the correct issue. Actually the thing is the datasheet in this section doesn't clearly state the requirement of a negative supply rail, nor the diagram shows it clearly, hence the misleading.

    If I had a negative supply rail available in my system, then I would have preferred to derive a -VREF net more simplistically. Anyways, I don't have that rail available in my system, hence I am giving up this approach and going ahead with a voltage output DAC of some other part number.

    Thank you for your time and support Katlyne.

  • 0 in reply to Vasudeo Bidve
    TI__Mastermind 28058 points

    Hi Vasudeo,

    I can see that the figure is misleading by not showing the supplies. Please let us know if we can help you select another DAC for your application. 

    Best,

    Katlynne