INA125: no changes in VAout

Part Number: INA125
Other Parts Discussed in Thread: LM7705

Hello Guys,

Good day.

Our customer is having an issue with the INA125. Below are the details and the attached schematic. Appreciate your inputs on this.


They feed device with 3V
They used an external 3v source to feed the load cell.
They press the load cell and they can read a difference of 1-4 mv between s+ and s- with the multimeter.
10k resistor were used to generate an amplification of 10.
The Aout pin of the device is always around 900mv and nothing changes even if they press the load cell.

They are using the same device in two different boards and they get the same result.

Thanks and regards,

Art

  • Art,

    The issue is that by connecting pin 15 to pin 4 you attempt to use internal Vref_out of 5V BUT internal Ref_Amp (see below) cannot drive the output to 5V while using 3.3V supply voltage, V+.  

    If you absolutely need a Vref of 5V, you must increase the supply voltage above 6.25V - see below.

  • Hi Marek,

    Customer is now feeding the load cell with an external 3v source and not with the reference voltage.

    Finally he has now connected pin 4 with pin 13, but is not using this voltage, just the external 3v. Even with this configuration he can not see any change in the VAout when he use the load cell.

    Also he has connected V- (pin3) to GND directly. Is that okay?

    Thanks and regards,

    Art

  • Art,

    I believe the issue is caused by the violation of the INA125 output voltage range. On a single 3V supply, the linear output range is from 0.3V to 1.8V (see datasheet table below) but with the output referenced to ground your customer attempts to drive it too close to negative rail.

    For 1mV-4mV differential input voltage, in Gain of 64, the output would have to swing between 64mV to 256mV, which is below the minimum specified of 300mV.

    To solve the issue you may either:

    1. Reference the output voltage above the ground by connect IAref (pin 5) to voltage of 250mV or higher

    2. Use a negative supply (pin 3) of at least -250mV 

    3. Power negative supply (pin 3) with LM7705 (-230mV negative charge pump generator) 

    4.  Increase the gain to at least 300 (RG=200phm) so for Vdiff=1mV Vout>300mV

  • Hello Marek,

    Upon implementing the proposed solution, below are the follow-up inquiries from the customer in verbatim.

    "1) I connected a resistor divider from vcc to pin 5 to get 300mV. When I read the value given by the divider I get correctly those 300mV, but when I connect the divider to the pin 5 I get 598mV

    2) I increase the gain from 10 to 16 (5k resistor ).

    3) now I am reading some changes in VAout but this:
    * When I don't do anything I read 600mv
    * When I use the load cell I read 1mv between s+ and s- and 602-604 mV in VAout..like a gain of 4...not 12

    ¿Why those 600mV?
    ¿Why this 598mV in pin 5?
    ¿Why that gain?"

    Thanks and regards,

    Art

  • Hello Marek,

    Just like to update regarding follow-up inquiries form customer.

    Thanks and regards,

    Art