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INA254: Contradiction between the theoretical calculation and practical values

Willie Neoh
Prodigy 30 points
Part Number: INA254


Tool/software:

Hi,

I have using INA254A2 to run a test. The gain written inside the datasheet for INA254A2 is 40 mV/A .

I have changed the position of R203 to R204 to enable my Ref1 and Ref2 are both connected to ground. Besides, I am lifting up the pin 10 to input a -2.5 V in to activate the board together with +2.5 V in the VS. 

Here comes the situation

When I input a 30 A to the IS+ and IS - , it gives me the output of 1.5 V, which supposing I should get 1.2 V. 

Based on theoretical calculation:

Vout = I X Gain

Vout = 30 A X 40 mV/A

Vout = 1.2 V

Can I get an explanation on this issue?

Thank you

Regards,

Willie

  • 0
    TI__Expert 5995 points

    Hi Willie,

    When you change pin 10 to be reference to -2.5V you need to make sure that all of your grounds are set to -2.5V including the ground of your current source. Inconsistent grounds could be causing this issue.

    The easiest way to test this would be to solder pin 10 back down to the board and apply the -2.5V to the GND test point on the EVM.  Also make sure the ground of your current source is the same as the -2.5V ground of the device.

    Please let me know if this fixes the issue.

    Best,

    Holly

  • 0 in reply to Holly R
    Prodigy 30 points

    Hi Holly,

    I have tried your suggestion, and it is not working as expected.

     

    When I pumped in 0 to 30 A, it can produce the correct output. However, there is no reading produced when negative current is pumped into the IS+ and IS- of the board.

     

    Please advise me what to do next.

     

    Thank you

     

    Regards,

    Willie

  • 0 in reply to Willie Neoh
    TI__Expert 5995 points

    Hi Willie,

    This is how I am understanding the setup, but please correct me if I am wrong:

    1. You are using our INA254A2 EVM
    2. The ground of the entire board is now at -2.5V
    3. REF1 and REF2 are tied to the ground of the board (-2.5V)

    If this is the case then the device is in unidirectional mode. I think the intention is for the REF1 and REF2 pins to be tied to mid-supply (0V), however, since they are still tied to the ground of the board and the ground of the board is now at -2.5V you are operating in unidirectional mode. In order to fix this you need to connect the REF pins to a voltage that is greater than the board’s ground. Section 8.4 of the datasheet provides more information on this.

    Please let me know if this helps.

    Best,

    Holly