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INA826: INA826 "+" input is floating

Noam Weidenfeld1
Intellectual 780 points
Part Number: INA826
Other Parts Discussed in Thread: MUX36D04

Hi,

I am using the INA826 in the following application.

4 x Analog 1 RTD.pdf

I would like to know what will be the INA826 output if I will leave the "+" input floating while the "-" input is connected to GND

Thanks

Noam

  • 0
    TI__Guru 60570 points

    Noam,

    Because of the INA826 input stage topology using PNP input transistors (see below), the input bias current comes out of the input terminal; for that reason, leaving the positive input floating will cause it to charge up close to positive rail. This, in turn, together with the negative rail connected to GND will result in the output to collapse close to positive rail (near the positive supply voltage used).

    You may simulated this condition using INA826 macro-model available on TI website - see below:

  • 0 in reply to Marek Lis
    Intellectual 780 points

    Marek Hi,

    I changed the input path and added an analog multiplexer before the INA826. See attached.

    Does it change the INA826 input impedance? How I calculate it?

    Thanks

    Noam

    analog input ii.pdf

  • 0 in reply to Noam Weidenfeld1
    Intellectual 780 points

    Marek Hi,

    I changed the input stage of the INA826 (Attached) and now I am using an analog MUX (MUX36D04).

    I need to know what will happen if the selected MUX "+ Input" is floating? What will be the output on ANA4? ("-" is connected to GND)

    One of your colleague suggested to add a pull-up resistor (10MOhm) on the INA826  "+" input. How it will affect the measurement results? Is there a better way to bring the output (ANA4) to a known state when the input of the MUX is floating?

    Thanks

    Noam

    Analog In.pdf