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DAC60508: Chip does not update outputs

Luciano Castagna1
Prodigy 40 points
Part Number: DAC60508

Hi

I'm trying to set outputs on my DAC60508 connected to a microcontroller but I'm not able to do it. Shutting down outputs by writing the config register works. Changing RefDiv and gain settings also works (I see the output dropping from 2.5V to 1.25V). However, writing on the output register does not change output voltage. I tried both modes synchronous and asynchronous (by setting LDAC bit on trigger register) and nothing. I tried writing DACx register 8 to F and even broadcast messages, but still outputs don't change. I connected a scope on SDI, CK and CS signals and all looked good, even the scope decoded the signal correctly (for example DAC1=0x08FFF0 or 0x080000). I tried different baudrates also although I assume the SPI communication works fine since writing into config and gain register work.

Any ideas? I also tried with another board but the result was the same.

  • 0
    TI__Guru 52663 points

    Hi Luciano,

    Try the following: Power cycle the device, do not write to any configuration registers, but write 0xFFFF to the BRDCAST register.  Do the DACs update?

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 40 points

    Hi Paul, I did it but still getting 2.5V in all outputs.

    I'm attaching a cope capture just in case I'm missing something (yellow: SDI  |  green: SCLK  |  Blue: CS  |  pink: SDO) 

  • 0 in reply to Luciano Castagna1
    TI__Guru 52663 points

    Hi Luciano,

    The ground bounce on that image looks pretty extreme.  Maybe there is digital crosstalk that is causing the device to misinterpret the SPI frame.  I have seen this on devices before, where a digital spur on CS will cause the device to think that there is a new CS edge, which will end the current SPI frame.  You can try adjusting the wiring so that they are shorter, as well as slow down the SPI clock, and add some small capacitance on the SPI pins to reduce the bounce.  Only 100pF should be a good start. 

    Can you share a schematic of your configuration?

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 40 points

    Paul

    By adding 100pF caps on all 4 SPI lines, I reduced the spikes by half, but still the outputs wouldn't change. Also, the fact that I'm able to successfully change configuration and gain settings would indicate that the SPI works, am I right?

    You can find below a section of the schematics.

    Regards

    Luciano

  • 0 in reply to Luciano Castagna1
    TI__Guru 52663 points

    Can you monitor the VIO and VDD supplies during the write command? Do you see a glitch or collapse when you set a new DAC value?  Also, you should have a capacitor on the REF pin to ensure the internal reference is stable.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    TI__Guru 52663 points

    Any updates?

  • 0 in reply to Paul_Frost
    Prodigy 40 points

    Paul

    I checked the power rails and they're fine. Before adding a cap on Vref, I decided to try a few more things since I didn't have a proper pad to solder the cap. 

    After a few tries I managed to get it working. The solution was to configure microcontroller SPI/CS ports with internal pull-up (they're configured with pull-downs).  

    I captured SCLK, SDI, CS and SDO signals and noticed no difference on the first three compared to what I got before. But SDO did change and remind logic high during SPI commands. 

    So, it's working now. 

    I got the pull-ups idea from the DACx0508EVM schematics. I reckon it'd be nice to have a heads-up on the datasheet regarding the pull-ups since they're so critical for DAC behaviour. 

    In any case, thank you for your prompts replies, it's good having someone on the other side giving some ideas and not just dead silence.

    Regards

    Luciano