AD8330 with AD8131

Hi everyone. Sorry for my english. Please try understand me:) I have a problem with AD8330.
I'm designing a DIY digital oscilloscope. I want to use a typical double j-fet circuit in input.
The j-fet are powered by +/-5V. Next AD8131 (+/-5V) are driving VGA AD8330(+5V). On all schematics I have
seen that the AD8131 has a two resistors: 24.9 and 49.9 ohm. My question is...can I connect the
inverting input to global ground and non inverting input trought the output j-fet without those resistors?
In my circut I can't connect the 49.9ohm resistior to ground, it's too big load for J-FET.

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  • Hi,

    I recommend the AD8129/AD8130 amplifiers for the box marked ???.

    Take a look at the AD830 data sheet (the AD830 was an earlier part with the same architecture as the AD8129/AD8130) -- it has a bit more theory.

    Don't forget:

    1. Check the slew rate of each part to ensure you can meet your full power bandwidth requirement

    2. Anti-aliasing filter

    3. Noise calculations and noise bandwidth

    4. Enough system gain to drive ADC to full scale (V_FS) at desired oscilloscope scale (V/DIV)

    Good Luck,

    Regards,

    Bob

Reply
  • Hi,

    I recommend the AD8129/AD8130 amplifiers for the box marked ???.

    Take a look at the AD830 data sheet (the AD830 was an earlier part with the same architecture as the AD8129/AD8130) -- it has a bit more theory.

    Don't forget:

    1. Check the slew rate of each part to ensure you can meet your full power bandwidth requirement

    2. Anti-aliasing filter

    3. Noise calculations and noise bandwidth

    4. Enough system gain to drive ADC to full scale (V_FS) at desired oscilloscope scale (V/DIV)

    Good Luck,

    Regards,

    Bob

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