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  • TI Thinks Resolved

INA125: Is this bridge sensor can work with this amp?

Part Number: INA125

Hello,

I'm using the INA125 with a load cell and it works good. 

Now i have another sensor which using the bridge method - but a little bit different -- it is an industrial pressure sensor like in the below link:

I'm already using the INA125 and I want to use the same chip for this sensor. is the INA125 will work as it should with this kind of sensor?

Thank you very much.

  

  • the sensor is
    86
    Compensated [TE Connectivity]

  • In reply to Itay Kalfa17:

    Hi Itay,

    The file in your first post does not seem to have gone through, but provided your pressure sensor is biased within the common-mode limitations of the INA125 and produces a differential voltage then there shouldn't be any issue interfacing with the sensor. If the impedance of the bridge is excessively large, then you may run into additional error from the bias current of the INA125, but this is 25nA typical so this shouldn't contribute anything significant. Let me know if you run into any issues!

    Regards,

    Zak Kaye
    Precision Amplifiers Applications 

  • In reply to Zak Kaye:

    Hi Jak,

    Thank you for the fast replay,

    ENG_DS_86C_A5.pdf

    I try to upload the spec again.. I noticed that in the load cell the spec talking about supply voltage and in the pressure sensor they are talking about supply current.

    I know that I can just put a resistor in series - but beside that, is it something that I should concern about?

    thanks  

  • In reply to Itay Kalfa17:

    Hi Itay,

    Yes it seems that all of the specs they give for the pressure sensor assume an accurate constant current source. You will want to create an accurate current reference for this. I believe the input resistance spec pf the sensor gives the bridge resistance seen by the source, and this can fluctuate from 2.5KOhms to 6.5kOhms. Assuming a constant 1.5mA source, your bridge voltage will be somewhere between 3.75V and 9.75V, meaning the common-mode voltage at the inputs of the INA should be somewhere between 1.875V and 4.875V. You need to make sure you have sufficient head room on your supply for this.

    Regards,

    Zak Kaye
    Precision Amplifiers Applications 

  • In reply to Itay Kalfa17:

    Hi Itay,

    the datasheet of pressure sensor is rather confusing. Does "input resistance" mean the resistance of bridge legs and "output resistance" the gain resistance?

    Well, if a constant current excitation of 1.5mA is needed you cannot work with a series resistor. Instead, you must provide a precise constant current of 1.5mA, because the voltage drop of this current across the bridge legs directly affects precision and linearity of your measurement.

    If the excitation current has to be 1.5mA to develop a full span measuring signal of 100mV across the bridge, then the resistance change of bridge legs is in the order of 100mV/1.5mA = 67 Ohm. Am I right? Whether the INA125 is suited, depends on your accuracy needs. Provided that you can calibrate out the offset voltage at zero pressure, then only the input offset voltage drift, the input bias current drift and the input offset current drift have to be taken into account.

    Kai
  • In reply to Itay Kalfa17:

    Hi Itay
    We haven't heard back from you, so we assume you were able to resolve your issue. If not, just post a reply below or create a new thread if this one has locked due to time out.

    Thanks
    Dennis
  • In reply to Dennis Goeke:

    Sorry for the late response, 

    I'm not fully understood from the answers if it will works.

    This is my current design:

    which i know it will works, but the INA125 will make my design much easier... 

     

  • In reply to Itay Kalfa17:

    Hi Itay,

    looks like you drive the sensor with a constant voltage and not with a constant current? But datasheet of sensor has all the data specified for a load current of 1.5mA. I guess you must drive the sensor with a constant current source pushing a current of 1.5mA through the sensor bridge.

    Have you contacted the manufacturer of sensor? What does he mean? Can he give you a schematic which shows how the sensor has to be connected?

    Kai
  • In reply to kai klaas69:

    Hi Kai,

    This is from TE application note:

    the input current can be calculate from this formula:

    I also added the application note for more details.

    thanks 

    APP-101_1 (002).pdf

  • In reply to Itay Kalfa17:

    Hi Itay,

    ah that's nice! And now you want translate this circuit into a circuit using the INA125?

    Kai

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