INA 125 amplification not beyond the reference applied !

Discussion in 'The Projects Forum' started by sanketh, Aug 13, 2014.

  1. sanketh

    Thread Starter New Member

    Jul 11, 2014
    Hey folks,

    Am a novice at electronics and new to this forum , so please try and correct my limitations .

    I have been working on a load cell circuit for a project of mine. However i am unable to go past the amplifying stage of the circuit . I am using the INA 125 burr brown amplifier whose datasheet can be found here : .

    Following the instructions in the datasheet i have made the following connection as shown in the jpg file .
    What i see in the datasheet is that the amplification is based on a single resistor Rg that is to be connected across pin 8 and pin 9 of the INA 125 IC .

    However given a certain input voltage , the amplified output that is read across pin 11 and ground doesn't seem to go beyond the applied reference voltage even if mathematically according to the value of Rg it is supposed to .

    For eg : The INA 125 is connected across 12V and when the input between pins 6 and 7 is 0.03V and the applied resistor is 6 ohms , the desired amplification is 10000 i.e the output across pin 11 and ground is supposed to be 30V but the voltage reads 11.3 V . The output just doesn't exceed 12V ( Vref) for any combination of inputs and resistors.

    Hopefully I have been clear regarding my doubt . Thank you in advance .

    P.S : Please forgive the bad image of the circuit diagram .
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Amplifiers do not create voltage, in a sense they direct it.

    So if you power an amp with 12V at most you will get out of it is 12V, and probably less.

    Like 11.3 volts.
    sanketh likes this.
  3. sanketh

    Thread Starter New Member

    Jul 11, 2014
    Hey erniem,

    Thank you for the quick response . So how do you suggest i get the desired amplification ? I mean to ask , now that i know the output cannot cross the reference voltage , what should I do if the output were ever to cross the reference voltage ? Should I balance the strain gauges such that the input they provide to INA 125 does not exceed a certain input value so that after a amplification of 250-300 , the output is well within 12V ?

    I was expecting a major flaw in my circuit diagram , din't know that I had got the concept of amplifiers wrong L:(
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    You must set the gain low enough such that (the most positive output from the strain gauge) X Gain < (INA Voh). The same can happen with (the most negative output from the strain gauge) X Gain > (INA Vol), assuming that the strain gauge can be deflected both ways...

    The INA data sheet will tell you Voh and Vol.

    Basically, you must initially balance the bridge to near zero so that the output of the INA is near half-way between Vol and Voh. That might be near 0V if you are running the INA on split supplies, or near Vdd/2 if running it on a single supply. That way, you have the maximum range for the amplified signal to swing without running into the rails...
    Last edited: Aug 13, 2014
    sanketh likes this.