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Original question:

DAC121S101: Output Collapsing

DAC121S101: Output voltage drop out.

Kathleen Jones
Prodigy 10 points
Part Number: DAC121S101

I'm experiencing a dropout in output voltage when I write to the DAC.  I noticed an issue so decided on a test.  I write the same value to the DAC.  I shouldn't see a change in the output.  During some of the writes the Vout drops to 0.

Yellow/top: DAC Vout = 2.0V

Green/2nd: Chip Select/SYNC

Red/3rd: Clock, 3.4MHz

Blue/bottom: data, 2400(9C3)

It's not a settling delay, it happens right at the 16th clock cycle.  I measured the 3.3V and I see no dropout.  It seems like the chip resets but then I captured it going high as shown:

Any suggestions?  Thanks

  • +1
    TI__Expert 7880 points

    Hi Kathleen,

    If you are only seeing this sometimes, not every time, my guess would be that it has something to do with your SPI communications. Note that the datasheet says the data is clocked in on the falling edge of SCLK. Your oscilloscope shows that the SDIN signal is acting as if the rising edge of SCLK is the important edge. Due to this, when the SCLK goes low, the data is shifting from low to high on the 12th bit. If the device reads this as "high" instead of the intended "low", you will go into power down mode according to the datasheet.

    I would suggest seeing if you can adjust your SPI communication to falling SCLK and then seeing if this issue continues.

    Thanks,
    Erin