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DAC104S085: Only selects first transmission

Jeridiah Welti
Prodigy 180 points
Part Number: DAC104S085

I am using a DAC104S085 in a product design.  On our initial hardware, I am only able to get the DAC to accept a single command and it ignores any command given after that.  

I have tried a variety of commands.

With a single DAC I am sending the following codes:

Rotate through power down modes:  0x7000, 0x3000, 0xB000.  I send one of these every 500ms and measure the voltage at the pin with a weak pullup to Va.  Whichever one I set first for a powercycle is the mode I see at the pin.  For example if I send 0x7000 first, I will measure ~0.9Vdc at the pins 2-5.  If I set it to 0x3000 first I will measure 3.3VDc at the pins.

Similarly, I tried setting 0x1000 and 0x1800 to write DAC A register and output immediately.  If I write 0x1800 first, the output will be ~1.6 VDC and will not change, and if I write 0x1000 first, the output will be 0VDC and will not change.

I have noticed when I set the outputs, that the clock and data lines become quite noisy until the DAC is powered down.  There is around 100mV of high frequency noise on those lines.  But this noise is not present when writing power down modes, so I'm not sure that is the root of the issue.

Any suggestions on what to look at?

  • 0
    TI__Genius 14220 points
    Hi Jeridiah,

    Thank you for your query. We are looking into the issue. In the meanwhile could you please send a scope capture of the waveforms?

    Regards,
    Uttam Sahu
    Applications Engineer, Precision DACs
  • 0 in reply to Uttam Sahu
    Prodigy 180 points

    The below scope traces show a transmission of 0x1800 which should be a write to the DAC register A, immediate write and update of outputs and set DAC A to 50% output.  There are 16 clk pulses, but they extend off the screen.

  • 0 in reply to Jeridiah Welti
    TI__Expert 5820 points

    Hi Jeridiah,

    Thanks for providing waveform captures. I agree that your second waveform capture is transmitting 0x1800 which should lead to the DAC output of 0.5*Vrefin. I want to make sure I understand the issue correctly. You can successfully write to the DAC and achieve correct output once, but then it does not respond after the first command? I assume you can cycle the power to the DAC and then write once again. Am I understanding this correctly?

    I noticed that you did not include the SYNC waveform in your captures. SYNC should be asserted low before the first clock edge, and asserted high after the 16th clock edge. If SYNC is asserted high before the 16th clock edge the write sequence will be ignored and the DAC output will not be updated. This could cause a problem like you are seeing. Can you confirm that you have the proper timing of the SYNC waveform?

    Regards,

    Garrett

  • 0 in reply to Garrett Satterfield
    Prodigy 180 points
    Yes, you have summarized the issue correctly. Yes, if I cycle power I can set it once again.

    The top trace shows the sync pulse. It is small, ~900 nS. The datasheet inidicates that a following sync pulse is not necessarily required. I have tried both a following pulse and not and neither has yielded response. When it is present it is long after the 16th clock(20uS).
  • 0 in reply to Jeridiah Welti
    TI__Expert 5820 points

    Jeridiah,

    Yes, this is correct. The following sync pulse is not required. Can you provide additional circuit details or a schematic if possible? What are the values of Va, Vref? Are you driving a load with the DAC?

    Regards,

    Garrett