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TINA/Spice/INA128: INA128 in single supply - Gain Problems

Armand Perin
Prodigy 45 points
Part Number: INA128
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

Hi dear supporting team,

i have a problem with my circuit which you can see below. I already read that i can use the INA128 in single supply. My problem is that the INA128 don't gain my Signal from DMS like i expect and want it (gain = 100). After trying a couple of things and still no better result, i hope you can help me. Thanks a lot!

  • 0
    Guru 140200 points

    Hi Armand,

    you should consider providing a reference voltage >0V to the REF pin of INA128:

    Also, your load R2 = 15R is way too low ohmic. Something in the kOhm range would be fine.

    Another cause for trouble could be the DMS itself. Some DMS can show a high imbalance causing a high offset voltage without any load.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 45 points
    Thank you for your fast answer!

    I understand your points and TINA-TI confirms them. But there is still one question: In the datasheet of the ina128 is written that in a typical application the load R2 is a low ohmic resistance like 12 ohm. Why is it in this case arround the kOhm range?
    It works out with a resistance of 1kOhm, thats just for me to understand the component "INA" better.
    Thanks, Armand
  • 0 in reply to Armand Perin
    Guru 140200 points
    Hi Armand,

    I can't find it. Can you show me where?

    Kai
  • 0
    TI__Genius 10850 points
    Hi Armand,

    Kai is correct. In regards to your comment about the datasheet recommending a low-ohmic R2, can you please share the specific section of the datasheet?

    -Tamara
  • 0 in reply to kai klaas69
    Prodigy 45 points

    At page 13 of the datasheet under the header "Typical Application". There's written:
    "The output is referred to the output reference (Ref) terminal which is normally grounded. This must be a low-impedance
    connection to assure good common-mode rejection. A resistance of 8 Ω in series with the Ref pin will cause a
    typical device to degrade to approximately 80dB CMR (G = 1)."
    Maybe i missunderstand it. Thank you two for your help.

    Is it possible to download this software somewhere which you used to simulate the ina with my paramters?

    Armand

  • 0 in reply to Armand Perin
    Guru 140200 points
    Hi Armand,

    I think you will find the software here:

    www.ti.com/.../ina-cmv-calc

    Go to the section "Software":

    www.ti.com/.../sboc452

    Referring to your question with the 8R. This resistance is meant to be inserted between the REF pin (pin 5) and signal ground. This is not the load resistance being connected from the output pin (pin 6) to signal ground.

    Beaucoup de chance et de succès!

    Kai
  • 0 in reply to kai klaas69
    Prodigy 45 points
    Thank you very much for your help and your patient with me!

    PS: Im German :D (only a French name).

    Armand
  • 0 in reply to Armand Perin
    Guru 140200 points

    Hi Armand,

    I'm German, too!! Haha...

    Dann wünsch ich Dir ein schönes Wochenende! :-)

    Kai

  • 0
    TI__Genius 10850 points
    This is resolved, will close the thread.