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dac8728 offset and gain problem

Ruth
Prodigy 40 points
Other Parts Discussed in Thread: DAC8728

Hi,

I have go to the point where I can  successfully convert digital to analogue using the dac8728.  I can write to its control registers and read the contents back however the DAC itself doesn't seem to respond to the settings.  For instance changing the offset from the default value seems to have no effect on the output signal.  I am using a 2.048 reference and trying to set a gain of 4 and expect to get +-4V output swing, however I get +-5V swing with clipping on one side so I suspect that the default gain of 6 might actually be being used.  I am powering the digital side with 3V3 first, then using the 3V3 the turn the +-5V analogue on using 2 x 2 FETs.  This switched 5V supply is then used to power the reference voltage ICs so it should be powering up in the right order.  All digital IO has pull ups except the nRST line which is driven from a microcontroller.  The nLDAC and nCLR are also connected to the micro (but also have pull ups).  Does anyone have any ideas why the DAC might be disregarding the control register settings please?  Or any idea about something else that might be causing the offset and high output swing please?

Many Thanks

Ruth

  • 0
    Prodigy 40 points

    Hi, sorry, the nRST has a pull down on it (shared with a flash device on a different schematic page so I didn't spot it - thought it was strange that I hadn't pulled it one way!).

  • 0 in reply to Ruth
    TI__Mastermind 46290 points

    Ruth,

    Thanks for following up and letting us know this was resolved. Let us know if you have any further questions.

  • 0 in reply to Kevin Duke
    Prodigy 40 points

    Hi Kevin,

    I still haven't got to the bottom of this.  I was just adding that the reset line did have a pull down on it and hence isn't floating and causing the problem.  I think I may have resolved the offset issue (uC problem). However I am still seeing +-5V when I expect +-4V.  Could you confirm that a Vref of 2 and a gain of 4 should result in +-4V please?  Is that where I am going wrong?  What else could cause this please?

    Many Thanks

    Ruth

  • 0 in reply to Ruth
    TI__Mastermind 46290 points

    Ruth,

    A 2V reference and gain of 4 should deliver an 8V output span centered about the offset DAC output voltage. Can you share a schematic? You might try probing the reference input to validate the actual voltage present at the reference input pins.

  • 0 in reply to Kevin Duke
    Prodigy 40 points

    Hi Kevin,

    I've sorted it out thank you - nothing wrong with the DAC with regards to the gain, just a ridiculous number if silly uC problems all either preventing the write to the gain register or overwriting it! So it took a while to debug.

    With regards to the offset, I was confused by the datasheet - I was trying to set the offset register to 0x0000 thinking that would put the DAC output span around zero.  So I've stopped doing that and am now leaving it as the default - which leaves it centred around zero.

    Regards

    Ruth