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TMS320F280049: Fast ADC inputs: What is the use of Op-Amp as buffer in between?

Wilson Pang
Intellectual 590 points
Part Number: TMS320F280049

Good day TI technical team,

Currently I am using two types of circuitry for Fast ADC sampling, with and without the Op-Amp as Buffer as shown below.

I notice that both circuitry are actually giving very close ADC readings since R711 and C702 are already calculated to meet 150nsec Sampling and Hold time based on TI reference manual.

May I know what is the reason that an Op-Amp as Buffer is recommended to be used?

I am just very curious if the high input impedance of the Op-Amp will help to improve anything? Is there any other benefit of using Op-Amp as buffer in between?

  • 0
    TI__Guru 60865 points

    Hi Wilson,

    In the non-op-amp case, circuit performance will depend on the impedance of the 'REF' input.  If the impedance becomes too high, the circuit will probably have issues settling. 

    In the op-amp case, characteristics of the 'REF' input will be mostly decoupled from ADC settling performance.   

  • 0
    Expert 1710 points

    Wilson,

    As someone who has been burned by this in the past, let me elaborate on Devin's reply a little. If your signal is coming from a high impedance voltage divider, or from a sensor with a high output impedance like certain photosensors, your ADC reading will be wrong unless you read it many times in a row. The op-amp takes that impedance out of the ADC path. You may need a special grade of opamp that's designed to drive larger capacitive loads. I had a difficult sensor problem caused by the engineer picking a high-current-drive op-amp, but not one designed for capacitive loads.

    Lloyd

  • 0 in reply to lslonim
    TI__Guru 60865 points

    Hi Lloyd,

    Thanks for the additional info, which I agree is correct and helpful!

    If you need help in the future selecting or validating an op-amp for the purpose of driving an ADC, make an e2e thread referencing the op-amp part # you are considering (vs the part # of the MCU with the ADC or the discrete ADC part #).  This will route the query to applications engineers in the op-amp organization; they should be able to best help you out. 

  • 0 in reply to Devin Cottier
    Expert 1710 points

    Devin,

    Thanks for the suggestion.

    Lloyd

  • 0 in reply to lslonim
    TI__Guru 60865 points

    Hi Wilson,

    Was your query resolved, or do you still have additional questions?  I'm going to mark this as resolved, but feel free to reply to re-open the thread.