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

Armand Perin

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!

over 6 years ago

0 kai klaas69 over 6 years ago

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 Armand Perin over 6 years ago 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 kai klaas69 over 6 years ago in reply to Armand Perin

Guru 140200 points

Hi Armand,

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

Kai

0 Tamara M Alani over 6 years ago

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 Armand Perin over 6 years ago 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 kai klaas69 over 6 years ago 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 Armand Perin over 6 years ago 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 kai klaas69 over 6 years ago in reply to Armand Perin

Guru 140200 points

Hi Armand,

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

Dann wünsch ich Dir ein schönes Wochenende! :-)

Kai

0 Tamara M Alani over 6 years ago

TI__Genius 10850 points

This is resolved, will close the thread.

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