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DAC101S101: Spurious-free dynamic range

Patroklos Anagnostou
Prodigy 50 points
Other Parts Discussed in Thread: DAC101S101

Hello,


Using the DAC101S101 we have measured parasitic frequency spurs besides the frequency we would want to output.

In the attached graph you can see the FFT of the digital values that are being written to the dac register (green) and the FFT of the dac output (red and blue). These spurs can be up to 30dB lower than the main frequency (3% of it!) and are moving in the range of kHz as the input signal moves by few Hz. At some points they superpose to each other and create the mentioned result while on others not (black curve, input signal just 15Hz higher, 4 smaller spurs).

Is this normal? What is the spurious-free dynamic range of the DAC?

  • The CS signal is not jittering
  • Fs is 150kHz. We tried varying it and this does not eliminate the effect but changes the parasitic frequencies
  • SPI clock 1Mhz
  • The supply is 3.3V and there is no fluctuation
  • There is no load on the DAC output
  • The phenomenon scales with proportionally to the input (no clipping or overflow effects)

  • 0
    TI__Genius 16890 points

    Hello Patroklos,

    There is no spec for SFDR for this device. 

    Since the VA pin is also the reference for the DAC101S101 it needs to be kept as noise free as possible.  Any traces (such as the SPI lines or others that contain signals) that could add noise to the VA pin should not run in parallel or cross the VA trace.  There should be a 0.1uF cap right next to the pin.  A larger cap, like a 1uF should also be close.  You may also need to add a series inductor in the VA line if there is noise.  All component grounds should go to a ground plane as should the ground pin of the DAC.

    A sample layout is shown below. 

    Mike