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OPA2620: Capacitor load

Bryan Deguire
Prodigy 70 points

Part Number: OPA2620
Other Parts Discussed in Thread: TLV2365

Looking for an alternate for the Analod Devices ADA4807-2, the Texas Instruments OPA2620 looks good but have a question about a capacitive load.  Is a unity gain buffer and a 10uF ceramic cap with no series resistor okay?  The data sheet discusses very lower capacitance values, 1nF needing a 10 ohm resistor.    

  • 0
    TI__Intellectual 2650 points

    Hi Bryan,

    For the OPA2620, while it might work I think there will be a lot of gain peaking without a series resistor. It can often depend on the output impedance of the amplifier, and given how the Cload curve in the datasheet starts to flatten around 10ohms for 1nF+ I don't think it is recommended in this case without a lot of gain peaking. 

    Some amplifiers can drive very heavy cap loads without a series resistor since that cap load basically becomes the new dominant pole. But for lower capacitive loads they still need an Riso since it interacts nearer to where the phase margin is an issue. 

    What are your system requirements for bandwidth with that 10uF cap? There is probably a better part that could work.

    Thanks,

    Evan

  • 0 in reply to Evan Kirkbride
    TI__Intellectual 2650 points

    I was just looking a bit closer and the ADA4807 datasheet has a couple tricks in their cap load drive section. One is that Figure 68 is in a G=+2 and not unity gain, plus its series resistance is actually 0.5ohms for 100nF which can make a big difference. Figure 69 also mentions "at 3dB peaking" so it can depend on how much gain peaking you want.

  • 0
    Prodigy 70 points

    Hi Evan,  Good point about the ADA4807 having gain of 2 in the C-load data.  My application is a buffer for a reference that has a 10uF cap on the output for an ADC, other buffer is between a mux and the ADC input.   A suggestion for a better part would be greatly appreciated.

  • 0 in reply to Bryan Deguire
    TI__Intellectual 2650 points

    Hi Bryan,

    I see, thanks for explaining. I was wondering why you needed a faster device with such a large cap load. But if one is driving an ADC and the other is a reference with the 10uF then that makes sense. What ADC are you driving?

    If it is just a reference for an ADC than I don't think adding a series resistor for stability would be bad to be safe. If you want to save on cost for footprint space then other potential devices are TLV2365. Its Cap Load curve trends down more, but its a less precise device.

    OPA2386 has some more bandwidth/slew rate, very low power but much larger input ib. But every device in simulation still has sharp gain peaking with such a large cap load without any Riso.

    Thanks,

    Evan

  • 0 in reply to Evan Kirkbride
    Prodigy 70 points

    Hi Evan,

    ADC is the AD7685, 16 bit SAR. 

    Thanks for your help with this issue should be all set for now.  

    Bryan

  • 0 in reply to Bryan Deguire
    TI__Intellectual 2650 points

    No problem, glad I could help!