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Original question:

INA225: Output truncation when measuring current

INA225: RINT value of INA225 and Slew rate

Lakshminarayana
Genius 3790 points
Part Number: INA225

Hi,

* I would like to know the Internal RINT resistor values which plays a vital role in selecting the Gains and also I notice that slew rate of INA225 is around 0.4V/us which seems to be less  for my application(In general).

* Is there any way that I can simulate my circuit with INA225 before i go into the Manufacturing.

*  I have few questions in my application using CT.

Voltage across the sense resistor will be +/- 0.4V (since CT outputs 50mA for Max Ip of 100A and Rs is 5.76Ohm).

I would like to know what happens when the input is to the amplifier is negative in case of unidirectional and Bidirectional?

Thanks.

  • 0
    TI__Mastermind 25165 points

    Hey Lakshminarayana,

    Thanks for considering to use Texas Instruments.  Based off the information I could gather from our datasheet, it looks like you can calculate the internal input impedance by sorlving Rint(1250/0.992-1250-10)=12500.  This would make Rint 155kohm (I will check with one of our design engineers to be sure).

    You can simulate your circuit by using one of our spice models found here.

    When current flows in the reverse direction and produces a negative voltage across your shunt , at the output you will see Vout=REF-|Vshunt|*gain.

  • 0 in reply to Patrick Simmons
    Genius 3790 points

    Hi Patrick Simmons,
    Haven't heard you from 2 days. Any feedback from the design engineer on those RINT Values?

    Thanks and Regards,

    Lakshminarayana K N.

  • 0 in reply to Lakshminarayana
    TI__Mastermind 25165 points

    Hello Lakshminarayana,

    Sorry for the late reply.  I found out from a design engineer that the RINT value should be 25kohm. From that value, if you use the example conditions provided in the datasheet (Rs = 10), the the gain error factor = 1250*25000/(1250*10+1250*25000+10*1250)=.99167.  

  • 0 in reply to Patrick Simmons
    Genius 3790 points

    Hi,

    Thanks for your inputs.

    May I know from where did you find this formula: " Vout=REF-|Vshunt|*gain ".

    Thanks.

  • 0 in reply to Lakshminarayana
    TI__Mastermind 25165 points

    Hello Lakshminarayana,

    I did not pull that formula directly from any specific place in the datasheet.  However all of our analog current shunt monitors work under the principle that the voltage across the shunt is multiplied by the device gain and added to the REF value.  I used a minus sign and absolute value brackets to emphasize that for a reverse current the output value will be less than the REF voltage.  The datasheet does provide some information on how current direction will generate an output above or below REF as shown in the snippit below.

    To further illustrate what I mean by reverse current, I am attaching an example below.

  • 0 in reply to Patrick Simmons
    Genius 3790 points

    Thanks for your reply.

    Is there any way I can simulate my circuit? I didn't understand how to use TINA S/W are there any other S/W?

    (Can you please provide me the link to Tutorials on TINA?)

    Regards,

    Lakshminarayana K N.

  • 0 in reply to Lakshminarayana
    TI__Mastermind 27715 points

    Hello Lakshminarayana,

    Our models are PSpice-compatible, so you can import them into any PSpice-based simulator such as Cadence or TopSpice. We also have reference material on how to use TINA:

    Best regards,

    Ian Williams
    Applications Manager
    Current Sensing