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ADS127L11: ADS127L11 with INA851 - Need help with bipolar configuration

Zane Lindstrom
Prodigy 10 points
Part Number: ADS127L11
Other Parts Discussed in Thread: REF6041, INA851,

Tool/software:

Hello,

I need some help with hooking up the ADS127L11 in a bipolar configuration with the INA851. I want the INA851 to be connected to +/- 15V and the ADC to be connected to +/- 2.5V. In another thread, an engineer posted the below pic, which helps, but it doesn't explain how to connect the instrumentation amp with this setup. Do I bring the common output voltage from the ADC directly into the amplifier? And what about this reference (REF6041) in the picture below? Does this mean that the input signal into the ADC can only go from -2.5V to +1.596V? Do I need to clamp the instrumentation amp to these voltages? Can/do I use this reference for the instrumentation amp as well? Thank you!

  • +1
    TI__Mastermind 39950 points

    Hello Zane,

    Welcome to the TI E2E community.

    Figure 9-9 in the INA851 datasheet shows the recommend connections between INA851 and ADS127L11 when using a single 5V supply.  Below is the configuration needed to support +/-2.5V supplies.

    The VOCM input  of the INA851 can be connected to the ADS127L11 VCM pin, or can simply be connected to ground since you are using bipolar +/-2.5V supplies.  The input signals of the ADC will be limited by the +/-2.5V supplies per the datasheet:

    The reference sets the maximum differential input voltage of +/-4.096V, where Vin-diff=+4.096V (AINP=+2.048V, AINN=-2.048V) and Vin-diff=-4.096V (AINP=-2.048V, AINN=+2.048V)

    Yes, the clamp voltage pins on the INA851 should be connected to the +/-2.5V supplies.

    You can use the 4.096V reference for additional signal conditioning in your system if needed.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 10 points

    Hi Keith, 

    Thank you so much for the help! I have a few more questions.

    1.) If I'm using extremely stable linear regulators (1uV RMS noise) for the -2.5V and +2.5V rails, is there any benefit to using a reference for the REFP and REFN inputs, or would it be fine to hook these rails up to the reference pins directly? And my signals will only go from -2.5V to 2.5V, so I'm guessing I can use the low reference range, which means I need to either hook up a 2.5V reference, or connect -2.5V to REFN and GND to REFP? 

    So for example, if not using a dedicated reference IC, I would hook my -2.5V rail up to REFN and GND to REFP to generate the 2.5V

    If using a dedicated reference IC, I would get the 2.5V version of this IC and connect the negative pin to REFN and the positive pin to REFP, which would essentially do the same thing

    Is that correct?

    2.) Do I need the 3 ohm resistor going into VDD1 on some drawings?

  • +1 in reply to Zane Lindstrom
    TI__Mastermind 39950 points

    Hello Zane,

    Yes, you can use a low noise LDO for the reference.  It depends on your accuracy requirements.  Most LDO's are only good to within 1% to 3%, some have higher errors.  Most references are 0.1% or better, plus, a reference will have very detailed specifications for temperature drift and long term drift, none of which an LDO will provide.

    If using a 2.5V reference, you can either connect REFN to -2.5V and and REFP to ground, or REFN to ground and REFP to +2.5V, either will work.

    Yes, for best performance, we recommend adding the 3Ohm resistor in series with the AVDD1 pin as shown in Figure 9-1 of the datasheet.  

    Regards,
    Keith