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DAC8811: Output Capacitance

Ted H
Intellectual 275 points

Part Number: DAC8811

The DAC8811 datasheet specifies 50pF typical, but code dependent.  Is the code dependence of the output capacitance characterized?

  • 0
    TI__Genius 14220 points
    Hi Ted,

    Thanks for your query. I am currently checking with the team. Will get back to you at the earliest.

    Regards,
    Uttam
    Applications Engineer, Precision DACs
  • 0 in reply to Uttam Sahu
    TI__Genius 14220 points

    Hi Ted,

    I checked with the internal team and found that we don't have any characterization data for output capacitance. Sorry for that.

    Regards,

    Uttam

  • 0 in reply to Uttam Sahu
    Intellectual 275 points

    I plan to use the the DAC8811 in a VGA with a less conventional configuration wherin the DAC is in the feedback path of the op-amp (see image below).  Given that the output capacitance is code dependent, but uncharacterized, can you tell me if it is still recommended to place a capacitor from the op-amp output to the inverting input for stability (as is suggested when the DAC is followed by a TIA?  If so, can you suggest a suitable capacitor value?

  • 0 in reply to Ted H
    TI__Genius 14220 points
    Hi Ted,

    The answer to your question is not straight-forward. Looking into the IOUT and VREF pins of the DAC, the resistance is going to be code dependent: the lowest resistance (5k) being at the highest code. This is the condition where the capacitance is also going to be the highest (~50 pF). You need to also consider the input capacitance of the op-amp and the parasitic capacitance at the inverting pin of the opamp. If the corresponding pole for these values falls inside the Unity Gain Bandwidth of the op-amp, you need a compensation capacitance. This will act as a zero to cancel out the pole and the value will also needs to be calculated accordingly. If the pole falls outside the UGB, there is no need for the compensation capacitor. The compensation capacitor may also slow-down the system at higher gains. Hence, in order to get consistent performance across the variable gain range, the op-amp selection will be key.

    You can use TINA to simulate this with actual parts. it will be difficult to predict the actual behavior qualitatively. The DAC can be modeled as an RC combination for this analysis.

    Regards,
    Uttam