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INA225: Can the REF pin be connected to the Vs pin？

yibing jiang
Prodigy 20 points
Part Number: INA225
Other Parts Discussed in Thread: TINA-TI, INA332

I used INA225 to monitor the current.  The Vs is 8V, IN+ and IN- is also about 8V.  When I set the REF to 1/2 Vs, it work well. But when I set the REF to Vs, it does not work. 

So I simulate the circuit with TINA-TI. A 10mV ,1kHz sine wave was added between  IN+ and IN-,  Vs was 8V, the result is same . The output was correct when REF was set to 1/2Vs.  When the REF was set to Vs, a half sine wave was excepted ,  but the output waveform  was strange. 

The datasheet described that REF is from 0 to Vs.  Can the REF be set to Vs really?  

  • 0
    TI__Mastermind 32395 points

    Hello Yibing,

    REF can absolutely be set to Vs. I believe the TINA model of INA225 has a bug where REF voltage must be different than the VS voltage. I mitigated this by setting REF pin to 7.9V.

    What do you mean exactly when you say the INA225 does not work when setting REF to Vs?

    Anyway, I would not use the model to validate this testing condition. I say this because even though it is valid to set REF to VS, sensing a sine wave voltage will mean the INA225 output will saturate to the VS rail when VIN > 0 and the model is not meant to perfectly model non-linear behavior. When the output of any amplifier is saturated, the INA225 will incur a overload recovery delay time in the output. So once VIN transitions to a negative voltage there will be a delay before VOUT comes out of Vs saturation and operates linearly. This is why you cannot achieve a perfect half-sine wave output.

    Worse yet, the swing-to-VS-rail spec for the INA225 can be up to 200mV max. This is much larger than the swing-to-GND which can be 10mV max for lowest gain. Thus, it is more advantageous to generate a positive sine wave at VOUT (REF=GND) as opposed to a negative sine wave because INA225 will come out of GND saturation faster compared to coming out of VS saturation.

    Please post back with more questions or detailed information as to what you are measuring and what gain the INA225 is set in if what I have explained does not resolve your issue.

    Sincerely,

    Peter Iliya

  • 0 in reply to Peter Iliya
    Prodigy 20 points

    Thank you very much for your reply.

    I checked my desgin again. The schematic and PCB was  correct.  But when I set  the voltage between IN+ and IN- from 0 to 20mV, the OUT(pin4) was always about  0V.

    I checked my BOM again.  I found a problem. The device mark which i used  was "B32".  I know INA225's device mark was "B32".  But in Ti website I found that the device mark of INA332 was also "B32". 

    Why INA332 and INA225 have the same device mark?

    How can I know the device type with "B32" mark?

    The device mark which I used was "B32  68PR", could you tell me it is INA332 or INA225?

  • 0 in reply to yibing jiang
    TI__Mastermind 32395 points

    Hey Yibing,

    The INA332 and INA225 have the same device marking because they're functionally similar and they come in the same package; however, they have completely different pinouts so it should be pretty apparent if the wrong one was populated. The given package marking cannot tell me what the device is. 

    Ideally, one should know which part they are using since they ordered the parts initially. If this device was working at some point with a grounded reference, then it should be easy which part it is.

    Sincerely,

    Peter Iliya

  • 0 in reply to Peter Iliya
    TI__Mastermind 34530 points

    Hi Yibing,

    We haven’t heard back from you for a while, and assume your issue has been resolved. If not, please post back and we’ll continue to support you.

    Regards, Guang