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INA826 Inst. amp. with different DC Differential input levels

jim macione
Prodigy 20 points
Other Parts Discussed in Thread: INA826

Im hoping to use 3 INA826 to measure a differential voltages over each capacitor of a total of 3 in series.    I made a model of the application in TI spice and have attached it.   In Tiny TI spice, the voltage on the lowest capacitor always measures about 0.07Volts lower than the cells in series above it.   In the application we are finding that there seems to be a larger offset voltage and we are not sure if this is related.  im trying to get an understanding of what is causing this and the best way to deal with it.  Is there an internal diode or level shift occurring?  I understand from the datasheet that the DC level on the Vref pin can alter the output but this dosent seem to be the case in the model.

 

Vref=0V (spice)

Vref=1V (spice)

Measured VRef=0

Cell1

1.91V

2.91V

1.81V

Cell2

1.98V

2.98V

2.06V

Cell3

1.98V

2.98V

2.06V

4034.INA826.TSC

  • 0
    Prodigy 20 points
    anyone have an idea?
  • 0
    TI__Guru* 93105 points

    Hello Jim,

    I believe the output voltage level difference you are observing between the one INA826 and the other two is due to a difference is due to differences in the input bias currents. The INA826 at the bottom of the 3-cell stack has its negative input at 0 V, while its other input and those of the other two INA826 amplifiers are at a higher, positive voltage. When the input is at 0 V, the bias current is about 10 % higher than at the other levels. You can see this input current difference in the TINA Spice circuit shown below.

    This current offset, which can be as much as 10 nA, develops a input voltage offset component across the INA inputs. This voltage offset is then amplified by the gain that the INA826 is set to, and results in a different output voltage than the other two.

    Regards, Thomas

    PA - Linear Applications Engineering