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ADS1278 with OPA or THS ?

R&D ELEC
Prodigy 50 points
Other Parts Discussed in Thread: ADS1278

Good morning,

I am designing a TMSC320C6748 based apllication with 16 Audio Diferential Input, so i've choosen an ADS1278, but reading its datasheet, page 38, i can see that i have to use OPAxxx or THSxxxx chip - depends wich datasheet's revision i read - ( anyway or maybe others Audio Operationnel Amplifiers) between Microphones and ADS1278. My question is "Why" ?

src : http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=ads1278&fileType=pdf

Diff. Discret filters on ADS1278's input is not enough ?

I don't understand "A true different driver or transformer (ac applications) can be used for this purpose" page 37.

Best regards,

Florent

  • 0
    TI__Mastermind 46290 points

    Hello Florent,

    Welcome to the E2E Forums! I'm happy to hear about your interests in the ADS1278! I'll try to go through your questions one at a time here, there's going to be some overlapping though.

    • Why must you use a amplifier between Microphones and ADS1278?

    For most audio applications input from a microphone is gained up considerably and the amplifiers are generally very low noise, no one wants to hear noisy audio. TI offers solutions for microphone pre-amplifiers that you may view here.

    • Differential Discrete filters on ADS1278's input is not enough?

    It's not so much about filtering as it is driving the ADS1278's sample and hold caps, though it is wise to filter out any extra noise at this stage anyway. Your signal may or may not be able to source enough current to charge the sample and hold cap internal to the ADS1278 to generate accurate data. This is true of almost any analog to digital converter unless the converter has built in amplifiers on the input stage.

    The ADS1278 is a quality part, and I don't mean to discourage you from using it, but you may want to consider looking at some of TI's Audio Converters and/or Audio CODECs as these may be better fits for your application. Some of these devices have built in amplifiers on their inputs and the devices in general are more tailored to audio applications.

  • 0 in reply to Kevin Duke
    Prodigy 50 points

    Hello Kevin,

    I have to apologize because i was not very clear.

    I use Knowles SPM0406HE3H-SB Microphones. They are already préamplified and the gain is configurable by one resistor and one condensator.

    So for my application, do you think THS chips are indispensable ?

    Thank you.

    Florent

  • 0 in reply to R&D ELEC
    TI__Mastermind 46290 points

    Florent,

    If your microphones have built in amplifiers and they can source enough current for the sampling caps in the ADS1278 then you should be alright. Make sure you put a capacitor on each of the ADC inputs to help charge the sampling caps and take some of the load off of your microphone pre-amplifiers, perhaps a little RC filter like you see on our EVM. You can put this system to the test relatively easily if you have a ADS1278EVM handy. On the EVM you can choose to switch out the on-board buffering amplifiers and drive the ADC with your microphones and check out your results.