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DAC8750: DAC8750 SPI issue

Philip Nubile5
Prodigy 70 points
Part Number: DAC8750

I have my first run of product using the DAC8750.  While all prototypes function correct, I have a few where SPI data does not go through.  In my code I continually update the control, config and DAC registers about once every 1.6mS.   If you put a scope probe on the clock line it starts working.  Viewing the scope waveform from one that works vs one that does not appears the same.  The SPI lines are directly connected to the host micro.  I am running the SPI clock at 16MHz (no change if I back it down to 4 MHz).   

These were assembled by hand.  Any chance of damage from assembly or do I have an electrical issue that is right on the edge ??

  • 0
    TI__Genius 14220 points
    Hi Philip,

    Thank you for your query. As you mentioned that it is a board-specific issue, the first thing to look for is assembly issues. Sometimes just re-soldering the board may work and you will be sure that this is something to take care during production. Sometimes, a clock line may not work if not terminated properly. I am not sure whether that is the situation in your case. However, it is a good idea to put a series termination on the SCLK line and make sure there are no ground plane breaks.

    Other things to look for are the voltage levels of the signals and the timing.

    Hope that answers your question.

    Regards,
    Uttam Sahu
    Applications Engineer, Precision DACs
  • 0 in reply to Uttam Sahu
    Prodigy 70 points
    Just a quick test on one that did not work ..... I put a 100 ohm series resistor on the clock line and that seems to make it work. Will test further.
  • 0 in reply to Philip Nubile5
    TI__Genius 14220 points
    I would suggest 33 Ohms so that the impedance matches a 50 Ohm PCB considering the output impedance of the SCLK source is around 20 Ohm. Please make sure you put the series termination close to the source.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    Prodigy 70 points
    Thank you Uttam. The series termination has solved the problem.