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DAC7821: DAC7821 Increasing value of the compensation capacitor

B C
Prodigy 60 points

Part Number: DAC7821
Other Parts Discussed in Thread: OPA2992

Hello,

I have in my design a DAC7821 followed by the OPA2992 using the bipolar output circuit shown in Figure 29 of the DAC7821 datasheet.   Due to limitations in our hardware, we are essentially generating sine wave using 10 points per cycle at approx 500KHz so we see significant quantization error when using 1-5pf compensation capacitor.   By changing the compensation capacitor C1 (in Figure 29) to 27pf, the output waveform is much smoother. C2 is also being used to provide additional filtering.

Given the datasheet recommends a 1-5pf capacitor, is it acceptable to use a 27pf capacitor here?  If it's acceptable is it possible to use an even larger capacitor here for additional smoothing of the waveform?

Thanks

  • 0
    TI__Guru** 100885 points

    B C,

    The short answer is yes you could use a larger capacitor, but I'd try it first. The capacitor is certainly there to increase your stability. However, the OPA2992 inherently has a very low capacitive load drive already and you might have a hard time keeping it stable regardless of this capacitance. Have you tried a different op amp for this application? Normally, I would expect the 1-5pF range of feedback capacitance to be enough.

    I did check through E2E and someone else had asked about this capacitance for stability. There's a reference to an app note for op amp stability, and in particular there's a discussion on lead compensation (starting on page 18) that describes the calculation for this capacitance. You can find the discussion here in Eugenio's second response:

     https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/271379/dac7821-positive-voltage-supply 

    And the app note here:

    https://www.ti.com/lit/pdf/sloa020

    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru** 100885 points

    B C,

    There's also another discussion of the capacitance in the following short app note. In that description, it's directly applicable to multiplying DACs.

    https://www.ti.com/lit/pdf/slyt300

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 60 points

    Thanks Joseph.  I noticed in this multiplying dac app note there are two different calculations for the pole f2.  The text has one version and the graph has a different equation.   Which formula is correct?

    Also for the DAC7821, will Rd just be 10K?

    Thanks

  • 0 in reply to B C
    TI__Guru** 100885 points

    B C,

    I think both app notes have issues with the equations, but are similar. The first app note misses the 2π term and ignores the op amp input capacitance and the second app note misses the feedback resistance. In the end, the calculations should be relatively similar as long as CF >> Cin. However, in your case CF is so small, that you might not be able to ignore it.

    Regardless RD will nominally be 10kΩ.

    Joseph Wu