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INA125: Multiple chips wired exactly the same and outputting different values

Luke Larson
Prodigy 40 points
Part Number: INA125
Other Parts Discussed in Thread: INA122

First of all I would like to say that I am not very technically apt with electronics but I am attempting to complete a design project that involves amplifying signal from a strain gauge Wheatstone bridge. I have soldered up 3 INA125 chips onto PCB boards identically and they are all giving me wildly different voltage outputs. Also, I should note that I am using the single supply wiring depicted in figure 6 of the datasheet.

One of the three circuits gives me the desired 2.5V drop. One is reading a drop of 0.75V. The last is reading a drop of 1.75V, respectively. I have no idea why this is happening and could use some assistance in figuring it out. I have walked around to every pin on each of the three chips and they are all being supplied with the intended amount of voltage. The only pin that varies in the 10/11 output pin.

Some more information about my circuit:

I am using 350 Ohm strain gauges/resistors in the bridge. Also, my gain resistor is 330 Ohms.

Could my soldering attempts be burning up the chips? Or are these chip especially susceptible to static damage? I am baffled why this is happening and am running out of troubleshooting ideas.

Thank you in advance...

  • 0
    Guru 140200 points

    Hi Luke,

    can you show a schematic? A schematic speaks a thousand words...

    Kai

  • 0 in reply to kai klaas69
    Prodigy 40 points

    Kai,

    Here is the Schematic and also the data sheet for the INA125 chip.

    Datasheet: http://www.ti.com/lit/ds/symlink/ina125.pdf

    Also, I have been thinking about this schematic some more and am confused how we can even achieve a voltage drop greater than 2.5V. A drop greater than 2.5V would mean that pin 10/11 has to output more that 5V, which isn't even possible with a singly supply of 5V? How is this supposed to work? Am I understanding this correctly?

    Luke

  • 0 in reply to Luke Larson
    Guru 140200 points
    Hi Luke,

    I guess your bridge draws too much current from the 2.5V reference voltage.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 40 points
    Kai,

    What do you mean by too much current? Why would the bridge current draw be affecting the total output of the opamp?

    Also, I have been thinking about this schematic some more and am confused how we can even achieve a voltage drop greater than 2.5V. A drop greater than 2.5V would mean that pin 10/11 has to output more that 5V, which isn't even possible with a singly supply of 5V? How is this supposed to work?

    Is there supposed to be a physical resistor between pins 11 and 5 (that acts as a voltage divider or something?) Let me know if I am not making myself clear enough because I feel like I'm confusing myself with all this :)

    Luke
  • 0 in reply to Luke Larson
    Guru 140200 points
    Hi Luke,

    the accuracy of voltage reference is specified for a range of 0...5mA. Have a look into the datasheet. This could mean that 2.5V / 350R = 7.1mA is too much current and short circuits some internal circuitry.

    This single supply circuit can be tricky, especially the 2.5V pseudoground node. You should not draw additional current from this node by external circuitry, because this will further increase the current to be sourced by the 2.5V reference voltage output.

    What do you connect to pin 5 and pin 11?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 40 points

    Hello Kai,

    There is no other external circuitry. Only the opamp and the bridge.

    Also, I am connecting my multimeter to pins 5 and 11 in order to read the voltage drop. Is this not correct? why is there a resistor symbol between pins 5 and 11, this has been confusing me for a while? I am beginning to question the legitimacy of the circuit schematic in the first place.

    Here is also a picture of the actual circuit I am working with (this is the one that sits at the intended 2.5 voltage drop). Is there anything obvious that you can see that might be wrong. I am a mechanical engineering student so all of this electronics is relatively new to me.

    In this circuit,

    Red goes to power

    Black goes to ground

    Green goes to strain gauge

    Yellow is my 2.5V pseudoground

    White is my 10/11 output pin

    Luke

  • 0 in reply to Luke Larson
    TI__Expert 6250 points

    Hi Luke,

    Could you measure the voltage of your 2.5V pseudoground/bridge reference with respect to your circuit ground? I suspect that the bridge resistance is too small, excessively loading the reference and causing its voltage to drop. If this is the case, consider using the circuit shown in figure 4 of the INA125 datasheet to boost the output current of the voltage reference. 

    One last thing to double check is that pin 2 (Sleep) is tied to pin 1 (V+) to enable the INA125. 

  • 0 in reply to Luke Larson
    Guru 140200 points
    Hi Luke,

    connecting the multimeter between pin 5 and pin 11 is correct.

    The resistor between pin 5 and pin 8 absolutely makes sense. This resistor is part of the internal instrumentation amplifier which is build following the well known "2-OPAmp instrumentation amplifier" topology. Many other InstAmps contain this sort of instrumentation amplifier. Look into the datasheet of INA122, for instance.

    Referring to your photo: I miss two things, the obligatoty decoupling cap across the supply voltage pins of INA125 and the forth bridge resistor.

    And, please follow the suggestions given by Alexander.

    Kai