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ADS131A04: ADS131A04 with external reference source

Lukasz Huchel
Expert 1420 points
Part Number: ADS131A04
Other Parts Discussed in Thread: REF5025, REF5020

Hi,

I have a question about ADS131A0x working with external reference voltage source. I use currently the ADS131A0x with negative charge pump enabled and my AVSS is tied to analog ground of the circuit. I need to change the design slightly and I am modifying the design to work without charge pump, AVDD connected to 2.5 V and AVSS to -2.5 V. 

- The modification on pin 11 is necessary and clear

- AVDD pin 9 is decoupled to AVSS on the device terminal.

- I got confused with the reference source. On page 31 of datasheet it is recommended to refer the reference source to AVSS which is now -2.5 V not GND. Also, the input voltage to the REF5025 is just 3.3 V with respect to GND. How should I understand it? I was sure I can keep the external Vref without any change (as 2.5 V with respect to GND) and provide it to ADC as previously referenced to ground. I attach recommended configuration below.

I will appreciate Your input.

  • 0
    TI__Guru* 92655 points

    Lukasz,



    First, If AVDD=2.5V and AVSS=-2.5V, you will not be able to connect the reference to ground so that the REFEXT=2.5V. The device requires some headroom so that REFEXT input is at least 0.5V below the AVDD line. This is shown in the Recommended Operating Conditions on page 8.


    Note that the reference negative input is AVSS.

    In the diagram you show, the buffer is limited. The op-amp (like any op-amp) won't be able to drive the signal all the way up into the rail. Both the input and output will likely be limited. This is the reason for a 0.5V headroom limit.

    If you want to use this bipolar supply (±2.5V) then you should connect the external supply to AVSS, so that the output of the reference is near GND. This will set the REFEXT to 0V, which should be 2.5V with respect to AVSS.



    Joseph Wu

  • 0 in reply to Joseph Wu
    Expert 1420 points

    Hi Joseph,

    Thanks for your answer. I had 2.5 V reference in my original design but I was using 3.3 V AVDD. Thanks for reminding.

    You have clarified my doubt regarding the ground of the reference source. Thanks.

  • 0 in reply to Lukasz Huchel
    Expert 1420 points

    I have just one last question. I can swap REF5025 with REF5020 to accommodate for the headroom of the of the buffer op-amp. However, REF5020 has 2.048 V rated output. Can I find detailed rail-to-rail specification for that op-amp (would 0.452 V headroom be sufficient)? If slightly less than 0.5 V is acceptable I can just change part number in the BOM.

    I will appreciate your help.

    Best.

  • 0 in reply to Lukasz Huchel
    TI__Guru* 92655 points

    Lukasz,

    To be honest, I don't have enough information on that. I don't think we have that specific characterization on the op-amp.

    It is likely that it would work fine if you made the replacement with the REF5020 for room temperature operation. If you wanted to test it, I would definitely test that over temperature.

    Joseph Wu