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LAUNCHXL-F28377S: DAC output does not go to zero

Arturo Jumpa
Prodigy 140 points

Part Number: LAUNCHXL-F28377S
Other Parts Discussed in Thread: C2000WARE

I have a LAUNCHXL-F28377S and I'm running sample code /C2000Ware_1_00_01_00/device_support/f2837xs/examples/cpu1/buffdac_ramp/cpu01/buffdac_ramp_cpu01.c. I noticed the voltage stays around 14mV when I command it 0 on the debugger (I change dacValto 0). The scope is wired to J3 pin 29 on my demo board. The only way The scope get's to 0V is if I change DacbRegs.DACOUTEN.bit.DACOUTEN = 0;. 

Do I have things wired incorrectly or is the board supposed to behave this way?

The lines that I have changed are:

#define REFERENCE            REFERENCE_VREF
#define DAC_NUM              DACB     // J3 pin 29


  • 0
    TI__Mastermind 28275 points
    Hi Arturo,

    The DAC has an offset but after Device_cal loads the factory trims, that offset should be less than -/+10mV. However, keep in mind that this is for midpoint offset code 2048 and not offset at code 0. The DAC performance characteristics in the data manual is only valid for the linear range. Nevertheless, do you also see the same issue on DACA?

    The signal you are showing on your scope is a bit noisy. If your scope has a high resolution mode and low pass filtering, can you turn those on so the signal is cleaner. You can also measure with a DMM instead.
  • 0 in reply to Frank Ankapong
    Prodigy 140 points
    When I use DACA I see a smaller offset of around 9mV. A Fluke multimeter shows 4.8mV. What could be causing these differences?
  • 0 in reply to Arturo Jumpa
    TI__Mastermind 28275 points
    This could be due to the scope probe and/or scope needing calibration. However, i think what is most likely going on is you have the scope sampling at a high frequency 4GSa/s which most likely means the anti-aliasing filter cut-off is also very high. This lets in more high frequency noise. Looking at your previous scope capture, the pk-pk on the signal alone is about 10mV which is problematic when you are trying to measure a 4.8mV signal.

    Since this is a DC signal, you could probably turn on averaging on the scope to give you a cleaner signal. Coming back to the discussion on the DAC offset, it looks like 4.8mV is within the data manual limits but as i mentioned before, we don't guarantee data manual performance outside the linear range.