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LMH6624: Stability at Av <10

xjonathanx xdufourx
Prodigy 240 points
Part Number: LMH6624
Other Parts Discussed in Thread: OPA843, OPA694, THS4271, OPA659

We use an opamp to scale the DAC's output into a suitable voltage range.  We're looking
at using the LMH6624 at a gain of 3x.  Apparently, there are some tricks to play for operating at lower
gains.


Is the LMH6624 stable at a gain of +3X ?

Is a resistor in series with the NI enough for stability ?

Thanks in advance

 

  • 0
    TI__Intellectual 1095 points

    Hi Jonathan,

    thanks for reaching out. I try to understand your stability problem but can you please provide more information:

    - what is the DAC's output for the op amp?

    - do you want to use an inverting configuration by saying "resistor in series with the NI"?

    - what is your desired output range?

    - which frequency is used in the application?


    Can you please share a schematic showing details how the LMH6624 is used?

    Thanks and regards

    Miriam

  • 0 in reply to Miriam Mesek
    Prodigy 240 points

    Hi Miriam,

    Thank you for getting back to me.  I've included the schematic; it answers your questions.
    I just need to make sure I can substitute the OPA843 with the LMH6624 to get more swing;
    the OPA843 is only spec'd for +2.8V min swing w/ 100R load (we need +3.6V into about 300R).
    The LMH6624 is the closest VF amp I could find that has the bandwidth and swing; I'm extrapolating
    that it can swing within 1.6V of the positive rail from the datasheet, does this seem reasonable?

    Thanks so much,

  • 0 in reply to xjonathanx xdufourx
    TI__Genius 15975 points

    Hi Jonathan,

    I would not recommend using the LMH6624 at G = +3 as it is only stable for gains higher than +10V/V. You could compensate noise gain of the device to make it more stable at G=+3. However, you end up with significantly increased output noise and reduced BW.

    Looking at your application circuit, you could use the THS4271/THR4275 VFB op amp to achieve +3.6V min into 300ohm resistor at G=+3. The OPA694 will be ideal for your application which can also achieve +3.6V min into 300ohm resistor.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 240 points
    Hi Rohit,
    Thank you for looking at my situation. Unfortunately, the physical component pcb arrangement is going to limit to package to a SOT-23 opamp, and it must be voltage-feedback to keep the offset current errors down. Is there any other options for G=+3X at around 300MHz BW ?
  • 0 in reply to xjonathanx xdufourx
    TI__Genius 15975 points
    Hi Jonathan,

    You could use the OPA659 which is available in SOT-23 and should meet G=+3V/V at around 300MHz BW.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 240 points

    Hi Rohit,

    This is a useful option.  We presently stock the OPA843, so we'll try to use it first.

    If the swing isn't quite making it, we'll look into the OPA659.  Thank you so much for

    the useful suggestion,

    All the Best,

    Jonathan