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ADS131E08: Bipolar Power Supply Configuration

Brett Gidge
Intellectual 370 points
Part Number: ADS131E08

I've previously posted a question on the TI ADS131E08 about AVSS and VREFN. I was told they need to be tied together. Here is my dilemma. The first spin of my board did not work because I violated the common mode range given my input signal levels (IN-P ranges from 0V to around 2.75V, also note that IN-N on all of my channels must be tied to GND because of the detection circuitry). See section 9.3.4.1 of the datasheet. Note, I originally had AVDD at 5V, AVSS at GND, VREFP at precision 3V, VREFN at GND. I need to be able to use this part because I need the simultaneous sampling. For the respin, I plan to change AVDD to 3.3V and AVSS to -1.7V (required to meet common mode range). VREFP will still be tied to precision 3V reference. VREFN will be tied to the -1.7V.

(1) The bipolar power supply connection section 11.4 states that the supply range is +/-1.5V to +/-2.5V. Is it OK to not have them symmetrical like I'm planning? I'm guessing what is important is that you meet the AVDD-AVSS range which is 2.7-5.25V. Please confirm.

Now the requirement to tie VREFN to AVSS means I need to create a precision -1.7V rail because it is the 3V precision reference and the -1.7V rail that create the part's VREF. Note, my measurements have to be super accurate - better than 0.2%. My 3V reference is 0.04%. Right now I'm planning on creating the -1.7V rail using an inverting opamp with in-loop compensation with 0.05% precision resistors. I have another thread going for the design of that rail. With the design of that new -1.7V rail, I'm worried about driving large capacitive loads with the opamp design.

(2) Do the VCAP caps have to go to AVSS or could they go to ground? If they have to go to AVSS then they present a bigger capacitive load to my -1.7V rail and might cause it to be unstable.

(3) Could I instead use a lower precision negative rail DC-DC supply to create the -1.7V for AVSS and then use my opamp design to create a high precision -1.7V rail for VREFN? The DC-DC supply would easily drive the required capacitance. The high precision -1.7V opamp rail would only go to VREFN. The only issue is that VREFN wouldn't be directly tied to AVSS. Is that OK?

Thanks for the help!

  • 0
    Intellectual 370 points

    No replies? It's OK because I figured out the proper way around this circuit design issue. I do have a different question but I'm going to create a new post.

  • 0 in reply to Brett Gidge
    TI__Guru* 92715 points

    Brett,

    I'm glad that you were able to solve this. Just in case, I thought I'd follow up.

    1. I've only seen symmetrical bipolar supplies, however, the way you have it set up should be fine. The only time I've seen asymmetrical supplies have problems is when the AVSS is pushed much further below the AGND. When this happens there are some digital level shifters that communicate between the digital section and the analog sections that may lose connection. From another similar device, I would guess that you need to keep AVDD above DGND by 2.25V.

    2. I would tie the VCAPs back to AVSS. This is the way it is recommended for bipolar supplies in the datasheet.

    3. I'm not sure if using a separated -1.7V for supply and a cleaner -1.7V for VREFN is ok. I would need to check with design for that one. However, I did want to point out that you don't need a precision -1.7V, you just need it to be moderately clean. I think you may have already come to this conclusion, but you would tie the negative reference to the bottom of another reference. For example it would look something like this:

    Regardless, I just wanted to follow up.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 370 points

    Thanks again Joseph. Yep that's exactly how I solved it.

  • 0 in reply to Joseph Wu
    Prodigy 90 points

    Hi Joseph, I am the 'software' guy working with Brett on this design. I did have a question about 'not needing to use a precision -1.7V, just moderately clean'. We are using an ADR443B voltage reference which is 0.04% accurate and 3ppm/C. If the ADC sees VREF = (VREFP - VREFN), why is it okay for the -1.7V to not be so accurate. Just for example, if the LDO in your diagram above was say 1% accurate, why wouldn't that be the weakest link in the precision of the VREF accuracy and now your VREF is only accurate to 1%?

    EDIT: Thinking some more, I guess if the function of our voltage reference is to precisely track 3V above its 'negative reference', the accuracy of the LDO wouldn't matter because it is just the difference. I imagine inside the ADC, the VREFP and VREFN charges some capacitor so it is only the difference between the VREFP and VREFN that appear on that capacitor.  The LDO could move around 1% but the difference would still be 0.04% accurate.

  • 0 in reply to Ken Bass
    TI__Guru* 92715 points

    Ken,


    I see that you've basically answered your own question. As you mentioned, the reference creates an accurate voltage measured from the reference negative voltage to the reference output. Therefore, the exact starting point doesn't matter as an accurate voltage. Again, the ADC uses the input from VREFN to VREFP, so you need to make a good measurement connection for these voltages.

    If you have any other questions, let me know.


    Joseph Wu