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ADS131E08: Does VREFN have to be tied to AVSS?

Part Number: ADS131E08
Other Parts Discussed in Thread: ADS131A04, ADS131M08, ADS1278, REF5030, OPA191, LM8261

I implemented the ADS131E08 on a board where I'm measuring signals (IN-P) that range between 0V and 2V. IN-N is tied to ground. I'm using an external precision 3V reference. Note, AVDD is 5V and AVSS is ground. The prototype isn't working properly because my common mode voltage, Vcm, doesn't meet the requirements in the "Input Common-Mode Range" section. My solution is going to be to respin the board with the following changes: set AVDD to 3.3V and set AVSS to -1.7V. I'm leaving my input circuits (IN-P and IN-N) the same and VREFP at 3V. This makes it so my Vcm falls within the required range. The question is do I have to tie VREFN to AVSS? In section 7.3 of the datasheet it shows VREFN as AVSS nominally. It doesn't give a range. Because my input signal is always positive, I was hoping to keep VREFN at ground so that my VREF (VREFP-VREFN) is 3V and not 4.7V which would keep my resolution the same. Appreciate any help.

  • Brett,


    I'm sorry about the input measurement not fitting in the input common-mode range of the device. This is common for many devices with an active amplifier used as a PGA for many of our ADCs. The input is limited by the topology of amp, similar to the front end of an instrumentation amplifier.

    For the ADS131E08, VREFN should be tied to AVSS, this is even stated in the pin functions table and it's why the recommended operating conditions table only lists the AVSS as the connection. VREFN should be connected to the lowest voltage of the device.

    Can you tie the negative connection of the reference to your new -1.7V source? This would be the best solution. What gain are you using for the PGA? If you are measuring an input of 2V, are you just using PGA=1.


    Joseph Wu

  • Hi Joseph,

    Yes I can tie VREFN to AVSS. I was just wondering if I had to. I really didn't want to have to increase my VREF (was 3V but will now be 4.7V) range if I didn't have to. Yes I'm using a PGA=1. That doesn't remove the Vcm range limit does it? Just wondering if the internal amps are bypassed when PGA=1.

  • Brett,

    Just to be clear, I was proposing that you tie the VREFN to the negative connection of the 3V reference. In that case, you would have the VREFN at -1.7V and the VREFP would be at +1.3V. What external reference do you plan on using?

    Even in PGA=1, that doesn't remove the Vcm range limit, in some other devices, we have the ability of completely bypassing the PGA when not used, but there isn't a good drop-in replacement for the ADS131E08.

    Joseph Wu

  • OK on the Vcm range not changing for PGA=1.

    I have a 3V precision reference that outputs 3V with respect to ground. That is what is going into VREFP. Then I plan to feed that 3V reference into an inverting opamp to create -1.7V. The -1.7V will be applied to AVSS and according to you I need to tie VREFN to AVSS so it will also be -1.7V.

    I can't have VREFP be at 3V above -1.7V because that won't give me enough headroom for my input voltage (which can vary between 0V and 2.7V) going into the ADC.

    This Vcm requirement is a giant pain. I have 8 inputs that range from 0 to 2V. I need to sample them simultaneously. If you have another ADC recommendation then I would love to hear it. Ideally an ADC that doesn't require a bipolar supply to do this would be great. I need to use an external reference because I need very tight tolerance measurements.

  • Brett,

    I'm sorry I didn't respond back last week, but I got pulled into an important project and away from E2E questions.

    I did want to say that there weren't many good replacements for this ADS131E08. Of them that are similar, there is the ADS131A04, which can be used with a 3V reference, and down to ground for an input. However, this is only a 4 channel device. Additionally, there is the ADS131M08, which also can be used down to ground and has 8 channels, but is limited to 1.3V for the magnitude of the reference. Most of this information is near the front of the datasheet in the Recommended Operating Conditions table.

    Another device to consider is the ADS1278. It should have the input range, but the reference is limited at higher clock speeds. I'm not as familiar with this device as some of the others in our product line. 

    One thing that I was thinking was that you should keep the ADS131E08 and use a reference that could be set to the -1.7V supply. If you set the negative input of the reference to this -1.7V and then use the REF5030 you could have a reference positive input at about +1.3V and you would keep a precision reference.

    For now, I'll close out this thread. Let me know if you need anything else and we can continue to work on this by posting back.

    Joseph Wu

  • Hi Joseph,

    No worries on the delay. Thanks for the reply. Let me give you just a little more background. I'm actually using two of the ADS131E08 because I'm doing 11 measurements that must be simultaneously sampled. The IN-N for each channel MUST be tied to circuit ground because of the detection circuitry. Eight of the signals range from 0 to 350mV and three of the signals range from 0 to just under 3V. My measurements must be accurate to better than 0.3%.  My Vref is a precision (0.04%) 3V reference tied between VREFP and VREFN. VREFN is tied to ground. The prototype had common mode issues because my AVSS was ground. My solution was to change AVSS to -1.7V. You said that VREFN and AVSS must be tied together. (Note, if this wasn't the case then I would be done because I would create a reasonable toleranced -1.7V rail and have VREFN tied to ground.) The issue is that now I must have a precision -1.7V rail since it makes up part of my Vref. I've prototyped it and it works but with the rework there are a bunch of white wires and I'm picking up noise on AVSS. I created the AVSS with an inverting opamp (TI OPA191) configuration using precision resistors. The problem is that I can't decouple the -1.7V rail because of opamp stability. Note, I put a 0.1uF decoupling cap on the rail and it crushes the noise. The opamp seems to be OK but I can't move forward with that when I respin the board. I'm just hoping that the -1.7V rail will be "clean" once it is properly put down on the PCB without any real decoupling. I've opened another TI thread to find out other ways to create a precision negative rail that I can decouple. If I find another option then I will most likely put both down on the PCB in the respin.

    Note, I did find your LM8261 opamp that can drive an unlimited amount of capacitive load. I was going to use it but then I saw that the offset voltage can be really high (like 5-7mV) which kills the precision of the -1.7V rail so I abandoned it.

  • This post is now being addressed here:

    https://e2e.ti.com/support/data-converters/f/73/t/893732

    Joseph Wu