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DAC60508: Mid-scale PORST (with 1.25V output voltage) using internal reference possible at all?

Frank Baettermann
Prodigy 10 points
Part Number: DAC60508
Other Parts Discussed in Thread: DAC63204

Dear all,

I've got a project in which I used a DAC60508MRTER because I was under the impression that it would start-up mid-scale using the internal reference of 2.5V. I expected 1.25V on all outputs. But now, first time operating the actual hardware I realized, that it's 2.5V output all over the outputs. And after reading the datasheet more carefully (and here) I'm quite puzzled why would the gain register default to a gain of 2? I'm just starting to talk to the devices so I did not read-out the actual DAC register but I assume to find the mid-scale code in there anyways? ;-)

I think I have read majority of posts concerning this DAC-series here... Excuse my frank question: Am I missing a big point somewhere? Is is possible to start-up with 1.25V at all?

Thank you in advance,
Frank

  • +1
    TI__Mastermind 25563 points

    Hi Frank, 

    The device starts up to mid-code (0x800). But with the internal gain set to 2, and reference divider set to 1, the default output voltage will be 2.5V based on the internal 2.5 reference. I believe these defaults were chosen with the assumption of a 5-V full-scale range where 2.5V is mid-scale. 

    Do you need all 8 channels? We have the DAC63204 that has integrated non-volatile memory. You could use the memory to configure the exact startup state you need. 

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Prodigy 10 points

    Thanks for replying Katlynne,

    actually I do need some channels: I do have 6x 8 channels daisy chained on my prototype boards. Also, I wanted a part that has higher resolution drop-in replacements available. Your 63204 is not chainable, if i Understand correctly.

    It would have been much nice if the datasheet stated somewhere, anywhere, more explicitly what "mid-scale" actually means in terms of "voltage after startup". I mean, if I was - I am not - using the part at 3.3V it would be even worse. I'd image pulling my (remaining) hair out trying to find the reason why all outputs have latched to VDD ;-)

    Maybe I should think about the a sample-and-hold solution with stacked 1:8 demuxes for 40 channels, SPDT analog switches for all buffers that default to a 1.25V reference on startup so I don't have to care about  DAC internals... So much more work and massively slower.

    Anyway, thank you!