What should I consider when selecting an op amp to drive an ADC?

There's so many different parameters to consider when picking an amplifier to drive and ADC, which ones are the most important?

  • That's a great question and one that takes a fair amount of time to address properly. I'll provide some of the top parameters you'll want to consider, but also recommend some excellent reading material so you can become an expert on the subject.

    Ideally the amplifier should not degrade the signal to be digitized in any way. Some of the key ADC specs you'll want to consider are the following.

    1. Signal to Noise and Distortion Ratio (SINAD, or S/N+D)
    2. Effective Number of Bits  (ENOB)
    3. Harmonic Distortion
    4. Total Harmonic Distortion
    5. Worst Harmonic
    6. Total Harmonic Distortion plus noise

    7. Spurious Free Dynamic range (SFDR)

    8. Signal to noise ratio (SNR)
    9. Analog Bandwidth (Full power, small signal)
    10. Two-Tone Intermodulation Distortion
    11. Multi-tone Intermodulation Distortion

    Now come the op amp specs, lets start first with the DC specs.

    1. Input/Output Signal Range
    2. Offset, offset drift
    3. Input bias Current
    4. Open loop gain
    5. Integral linearity
    6. 1/f noise (voltage and current)

    Now the key AC specs. These are highly application specific

    1. Wideband noise (voltage and current)
    2. Small and Large signal Bandwidth
    3. Harmonic Distortion
    4. Total Harmonic Distortion (THD)
    5. Total Harmonic Distortion+Noise (THD+N)
    6. Spurious Free Dynamic Range (SFDR)
    7. Third Order Intermodulation Distortion

    Yes I know that's quite a lot to consider, but that's what you asked for and that's why its very challenging as well. Fortunately Analog Devices has a variety of tools and technical literature to help you pick and chose the right amplifier for your application.

    Analog Devices Circuits from the Lab (CFTL's)feature a collection of op amps and ADC's that have been built up and tested, so you don't have to do it. These CFTL's help reduce design time and lowers risk.

    We also have lots of design tools to help you find and analyze op amps for your circuit you can find them listed here.

    And last but not least we have an extensive library of technical documentation available for FREE. Many of our best selling text books are also available as free downloads.

    The Data Conversion Handbook chapter 6 discusses in detail the various considerations for op amps used to drive ADC's. Also available for FREE is the Op Amp Applications Handbook each of these is chock full of incredible useful and easy to read material.

    I hope this helps.

    Regards

    John

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