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ADS127L11: Differential Gain Amplifier for 24 Bit ADC

Benjamin Bawkon
Expert 1100 points
Part Number: ADS127L11
Other Parts Discussed in Thread: PGA280, , THS4551, OPA187, INA821, INA828, TINA-TI

Good Day,

I've got a product about to go to production using the new ADS127L11 24 bit ADC with a PGA280 in front of it for signal gain.  For the speed, accuracy and noise we need in this system, it's a perfect match, giving us the wide input common mode range that we need with excellent DC accuracy across a wide temperature range.

I've got a new design coming up that is going to be more demanding with regard to noise.  The PGA280 is a great amplifier, but it's noise contribution seems to prevent more than ~15 noise free bits at reasonable gains.

In order to maintain the excellent dc accuracy, low temperature drift and wide common mode input of the PGA280, while reducing noise significantly, does TI have any suggestions on a signal chain?  I need adjustable gain, but I'm not at all opposed to using analog switches with different precision resistors for that purpose.

I've considered a OPA187 -> THS4551 with 4x analog switch for gain -> ADS127L11.  Thoughts?  I'm also somewhat open to other ADCs, but we're actually really comfortable with the performance of this part.
Currently using REF5025, but I'm also open to other references - though we've found the reference to be negligible compared to the amplifier with regard to noise.

As always, appreciate any insight,

Ben

  • 0
    TI__Mastermind 23050 points

    Hey Ben,

    I think using the THS4551 as the single-ended to differential conversion and driving amplifier and manipulating the gain with an amp like the OPA187 is a great choice. First though I would ask how you are operating the ADS127L11 and what performance you need in your system. While the ADS127L11 is our newest converter, depending on the performance you need it might be easier to use one of our devices with an integrated PGA. 

    Best,

    Zak

  • 0 in reply to Zak Kaye
    Expert 1100 points

    Zak,

    Well, I switched gears a little.  The design requirements are difficult for this one.  Low noise, high input impedance, low input bias current, wide temperature range, wide common mode, high gain, fast settling (multiplexed system).  I know, I'm not asking for much.

    After experimenting with a few different ideas, I've come up with an amplifier that simulates really well.  Almost.  It simulates great with INA828, but that part is too physically large for me.  I can find no good reason in the datasheets that would lead me to believe INA821 isn't also perfectly acceptable, but that simulates terribly.

    Any chance we can take this to email?  I'd like to share some TINA-TI designs with you, but they're considered pretty company proprietary and I can't post them here.

    At this point, I understand this probably belongs in the Amplifiers forum, so if you would rather move it over there, I absolutely understand.

    Thank you very much,
    Ben

  • 0 in reply to Benjamin Bawkon
    TI__Mastermind 23050 points

    Hey Ben,

    I've sent you an email if you'd like to share the designs directly!

    Best,

    Zak

  • 0 in reply to Zak Kaye
    Expert 1100 points

    Zak,

    I've taken the question over to the amplifiers forum since it really seemed more appropriate there.  Sorry for the trouble!

    Best Regards,
    Ben