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DAC7718: oscillations on the reference input

Bart
Expert 6310 points
Part Number: DAC7718

Team,

we've observed oscillations on the reference input of the DAC (in amplitude, around 100mV, few tens of kHz). Removing the 1uF capacitor in the Vref, the oscillations were gone.

Afterwards we've observed similar oscillations also on the analog outputs of the DAC as well - here removing the output capacitor 10nF seemed to resolve it.

Can you advise if this is normal behavior?

Can you check the attached schematics?

Thank you,
TI Customer

sch.zip

  • 0
    TI__Guru 52683 points

    Hi Bartosz,

    I think this is an issue with the reference, not the DAC.  As you can see in the MCP1501 datasheet, the reference cannot have a capacitive load greater than 300pF without and isolation capacitor.  The large capacitor you placed on the pin is resulting in oscillations of the output amplifier of the reference.

     

    Thanks,

    paul

  • 0 in reply to Paul_Frost
    TI__Expert 6310 points

    Hi Paul,

    thanks for your feedback. On the output of the DAC we see the oscillation as well - this is not related to the reference.

    Despite no oscillation on Vref anymore, we see the oscillation as on the below picture. The green part is the DAC generated sine in 0-12V. The oscillations disappear when DAC is set to around zero.

    The yellow part is the constant 10V (oscilloscope channel capacitively coupled). Both graphs show oscillations in different phase and similar frequency.

    Removing the 10n cap from the DAC output eliminates these oscillations, though according to customer, the behavior of the DAC7718 is not correct.

    Please advise.

    Thanks,
    Bartosz

  • 0 in reply to Bart
    TI__Guru 52683 points
    Hi Bartosz,

    I think this load on the DAC is causing the oscillation. Removing the cap will help, but I think the ferrite bead is causing problems too. Can you short the bead and see if the oscillations stop?

    Thanks,
    Paul
  • 0 in reply to Paul_Frost
    TI__Guru 52683 points
    Hi Bartosz,

    Any update?
  • 0 in reply to Paul_Frost
    TI__Expert 6310 points
    Hi Paul,

    we've been doing such experiments - the bead didn't have a visible impact on the oscillations. The load of the DAC was only the oscilloscope probe (10MOhm). Simple RC filter (22Ohm + 1uF) was blocking the output oscillations fairly well, but that is not a solution to the problem.

    Thanks for your advice on this - we're looking forward to solve it.

    Kind regards,
    Bartosz
  • 0 in reply to Bart
    TI__Guru 52683 points
    Hi Bartosz,

    Increasing R33 should also help isolate the capacitive load from the DAC output buffer. Can you try increasing the value to 100Ω?

    Thanks,
    Paul
  • 0 in reply to Paul_Frost
    TI__Expert 6310 points
    Hi Paul,

    increased the value to 100Ω - the oscillation stopped. In configuration 51Ω and 100nF there is also no oscillation.
    Is that normal behavior?

    Thank you
  • 0 in reply to Bart
    TI__Guru 52683 points
    Hi Bartosz,

    We specify capacitive load stability for the DAC7712 as 500pF maximum. This specification should be interpreted as the largest capacitor that can applied to the output while keeping the output stable. Remember that as this is a buffered output DAC, you basically just have an opamp on the output pin. A large cap can make the system unstable.

    You did the right thing by adding the resistor between the load, but it still might be too much for the DAC. You might have also found a particular combination where DAC is unstable. It could be that the a lower resistor value can be used with a larger cap as well, like with the 51Ω and 100nF.

    Unfortunately we do not have a SPICE model for the DAC, which would be able to help you pick a good value.

    Thanks!
    Paul