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Wrong gain with ina827

Rasmus Bunkenborg
Prodigy 30 points

Other Parts Discussed in Thread: INA827

I am currently working with INA827 but it seems that the gain is incorrect. I have it coupled in single supply with 24V, the gain resitor is 845 ohm and the ref is at 0V.

In stead of having a gain of 100V/V it amplifies 176V/V. The gain error remains constant for all intended amplifications.

What are the possibilities for errors?

What can I do to determine what the error is?

I have checked:     Correct IC(it says IPSI on the package)

                                 Suppy voltage

                                  Gain resistor

                                  Correct input voltage

Any and all help is much appreciated

  • 0
    TI__Guru* 93105 points

    Hello Rasmus,

    The gain of the INA827 is internally trimmed and the error should not exceed the 0.4 % (G = 10 to 1 kV/V) listed in the data-sheet Electrical Characteristics table. Therefore, I would not expect such a large gain discrepancy to be caused by the INA827.

    Current shunt monitor applications such as yours are prone to errors in the current sensing circuit. The sense resistor is only 30 milliohms and it is easy to accumulate additional resistance in the high current, shunt resistor path. If the resistance in that path doubles to 60 milliohms and the lamp current is as expected the end result would be as if the INA827 is operating with a gain of 200 V/V. You need to take into consideration the shunt resistor connection resistance, its tolerance and temperature coefficient. If the shunt resistor heats when current is flowing through it and it has a positive temperature coefficient then the voltage developed across it will be higher than expected. Again giving the illusion of an INA827 gain error.

    One other thing to check is to make sure the INA827 is stable in operation. If by chance the INA827 is oscillating, the specified electrical characteristics may no longer hold. Use a wideband DSO and hi-Z, low-C probe when checking the stability.

    Regards, Thomas

    PA - Linear Applications Engineering 

  • 0 in reply to Thomas Kuehl
    Prodigy 30 points

    Hello Thomas

    Thank you for your answer. I had not considered that but I checked and you were correct. 

    I owe you a beer:)

    Regards

    Rasmus

  • 0 in reply to Rasmus Bunkenborg
    TI__Guru* 93105 points

    Rasmus,

    My pleasure to have assisted with your INA827 application.

    Thomas

    PA - Linear Applications Engineering