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INA128: Variation in voltage at Vref terminal

Denny Varghese
Prodigy 160 points
Part Number: INA128
Other Parts Discussed in Thread: REF3112, REF3312

Hi Team,

We are using INA128, Instrumentation Amplifier in our design to measure the voltage across a Wheatstone Bridge - Load Cell.

The load cell output terminals are connected at VIN+ and VIN- terminals of the Instrumentation Amplifier INA128.

We have used 2.5V and a resistor divider with two 12K_0.1% resistors as shown in the attachment, to apply a reference voltage of 1.25V

But when measuring, we see that 1.43V is present at VREF terminal.

Also, this reference voltage is changing whenever the load voltage across the Load cell changes.

Could you kindly help in understanding the variation in reference voltage and if it doesn't remain constant?

We are using the below relationship where we assume Vref = 1.25V.

Vout (V) = ((VIN+) - (VIN-)) * Gain + Vref

Thanks,

Denny

  • 0 in reply to Denny Varghese
    TI__Guru 62550 points

    Since the reference pin of INA128 is connected to a difference amplifier, you must run the Vref pin with a low impedance source or Vref will be changing with the input voltage caused by changing current in 40k resistor (SEE BELOW).  

    Thus, in order for the reference voltage to be constant at 1.25V, you must add a buffer between the resistor divider tab and Vref pin -see below.

  • 0 in reply to Marek Lis
    Prodigy 160 points
    Hi Marek,

    Thank you for your valuable suggestion. I will definitely add a buffer as recommended since buffer using opamp will have low impedance.
    Also kindly help me in finding the required "impedance range" or "limit" since it's only mentioned in the datasheet to have a low impedance source?

    Thanks,
    Denny
  • 0 in reply to Denny Varghese
    TI__Guru 62550 points

    Denny,

    Since the close-loop output impedance, Rout, of an op amps is a function of the open-loop output impedance, Ro, and a loop gain ( ), it is very low (Zout<<1ohm); thus, any op amp in a buffer configuration will have a low enough impedance and can be used to drive Vref pin of INA128, and the only other consideration is that it meets the required DC precision like voltage offset.  Alternatively, one could use an appropriate reference voltage to drive the Vref pin so long as it can sink and source current - see REF3312 or REF3112 (Vref=1.25V).

  • 0
    TI__Genius 10850 points
    Hello Denny,

    We haven't heard from you. Hopefully we were able to resolve this issue for you. I am going to close this thread now.

    -Tamara
  • 0 in reply to Tamara M Alani
    Prodigy 160 points
    Hi Marek,

    I have added a buffer as suggested and the issue has been completely resolved.

    Thank you very much for all the information provided.

    Best Regards,
    Denny