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INA250: Output reading too high

William Rodgers
Prodigy 80 points
Part Number: INA250

I have purchased 2 pieces of the INA250A4PWR from the TI store.  I have the part in a test jig.   I have a 5V device supply with a 0.33uf cap to ground.  The reference pin is grounded. The output is terminated in a 10K resistor.     SH+ is connected to VIN+ and SH- is connected VIIN- as shown in your simplified schematic.

The IN+ voltage is 10V and the IN- is connected to ground with a 47 ohm resistor, drawing about 200 ma.  However, the output is reading 5 volts.  I would expect an output  voltage of about (0.2 A)(2V/1A) = 0.4V.    I have 2 devices and both read the same.   Am I missing something?

  • 0
    TI__Mastermind 27715 points

    Hi William,

    What is the voltage at IN-? If the differential voltage is too large, it would saturate the INA250A4 output at 5V as you've seen. Based on your description and a quick sim/calculation check, it sounds like there's nothing wrong with your approach.

    Is the 47 ohm load resistor high-power? With 200mA through it, by I*I*R it would need to handle .2 * .2 * 47 = 1.9W or more.

    Can you provide a schematic of your system as wired?

    Best regards,

    Ian Williams
    Applications Manager
    Current Sensing

  • 0 in reply to Ian Williams
    Prodigy 80 points

    Hi Ian,

    Thanks for the reply.  I have attached a Power Point schematic.  The current drawn was verified on the milliamp meter in the HP supply.  The 47 ohm resistor was under-rated, but I only attached it for a few seconds 

    One thing I may have done that could affect the results was that I wired the two TSSOP's to separate adapter boards.  Then I added pins to the adapter boards to test them on a breadboard test jig.   With one chip, pins3 and 4 are shorted at the pins on the adapter board.  I did not worry since these pins are shown connected together internally.  Maybe I that could be problem.  However, I do not think that I shorted these pins on the second adapter board.

    I tried increasing the load to about 67 ohms.  This caused the current to reduce to 140ma.  The output finally came out of saturation and registered 4.80 volts.

        ... Bill

  • 0 in reply to Ian Williams
    Prodigy 80 points

    Hi Ian,

    Thanks for the reply.  I have attached a Power Point schematic.  The current drawn was verified on the milliamp meter in the HP supply.  The 47 ohm resistor was under-rated, but I only attached it for a few seconds 

    One thing I may have done that could affect the results was that I wired the two TSSOP's to separate adapter boards.  Then I added pins to the adapter boards to test them on a breadboard test jig.   With one chip, pins3 and 4 are shorted at the pins on the adapter board.  I did not worry since these pins are shown connected together internally.  Maybe I that could be problem.  However, I do not think that I shorted these pins on the second adapter board.

    I tried increasing the load to about 67 ohms.  This caused the current to reduce to 140ma.  The output finally came out of saturation and registered 4.80 volts.

        ... Bill

  • 0 in reply to William Rodgers
    TI__Mastermind 27715 points

    Hi Bill,

    Can you please re-attach the schematic? I'm not seeing it come through on my end. Also, have you measured the voltage at IN-? That's key to figuring out what's going on.

    Best regards,

    Ian Williams

  • 0 in reply to William Rodgers
    Prodigy 80 points

    Ian,

    I cannot get the power point schematic to post.

      Bill

  • 0 in reply to William Rodgers
    Prodigy 80 points

    INA250A4 test jig.pdf

     

     

     Ian,

    I tried again to attach by copying Word and Power point into your receiving space with no luck.  I tried a PDF and had no luck??

    Anyway, I did some measurements.  To the limits of my multimeter,  IN+ = 9.08 volts.  IN- = 9.07 volts.   Load was 57 ohms and drew about 155ma.  Output was 5.07 volts, and pin 10 'VS' was 5 volts.

    I just pasted the schematic below and it seems to be coming though??

    Let's see if this works.  ... Bill

     


     

     

     

     

     

  • 0 in reply to William Rodgers
    TI__Mastermind 27715 points

    Hi Bill,

    The PDF version did come through, thank you. I don't see anything wrong with the connections as drawn. At this point what seems most likely is a setup issue. Can you please do a couple more measurements to help narrow down the issue?

    • Measure pin 7 (REF), verify it measures 0V
    • Disconnect the 10V supply and load from the IN- and IN+ pins. Short all IN+ and IN- pins to GND. Verify that pin 9 (OUT) = ~0V.
    • With all the IN- and IN+ pins still grounded, apply 1V to pin 7 (REF). Verify that pin 9 (OUT) = 1V.

    If any one of these steps don't give the expected result, there is an issue with the setup.

    Best regards,

    Ian Williams

  • 0 in reply to Ian Williams
    Prodigy 80 points

    Hello Ian,

    Well 2 out 3 is not bad! 

    *  Pin 7 (Ref) tested 0V with no current through IN+ and IN-.

    * With the 10V disconnected and IN+ and IN-  grounded, pin 9 measured ~0 volt

    * However, with IN+ and IN- grounded, and 0.979V applied to pin 7 (REF), pin 9 (OUT) measured only 0.367V ????

    I guess the clue is related to how I am using the reference pin ?

    Best regards,

    Bill

  • 0 in reply to William Rodgers
    Prodigy 80 points

    Hi again Ian,

    I had a cockpit error.

    I dropped the 10v supply to 1 volt, and the 5 volt regulator went out of regulation.

    I raised the voltage so that the regulator did not drop out, andI used a battery on  pin 7 (REF) at 1.37V.  Pin 9 (out) measured 1.37V.

    Sorry to mislead you.

    Bill

  • 0 in reply to William Rodgers
    Prodigy 80 points

    Hi Ian,

    I found the problem.  On the prototype breadboard, I had jumpered from pin 16 (IN+) to pin 12 (VIN+) assuming that pin 16 (IN+) and pin 13 (SH+) were connected directly together inside of the chip.  When I moved the jumper that was between pin 16 and pin 12, to go between pin 13 (SH+) and pin 12 (VIN+), the results became much more reasonable.  I am reading less than twice the expected value.   I read 0.415V versus the calculated 0.280V for  ~140ma of current which is reasonable for the test setup.

    Thanks for all the help,

     ... Bill

  • 0 in reply to William Rodgers
    TI__Mastermind 27715 points

    That'll do it! Glad it's working better now.

    If you short any of the IN+ or IN- pins directly to the VIN+ or VIN- pins, you lose your Kelvin connection for sensing the shunt voltage. If you did the same thing on IN- and VIN-, you should correct those as well. 

    Part of why you're still seeing inaccurate performance is likely due to the breadboarded and jumpered nature of your setup, where the parasitic impedances of the wires is significant compared to the internal shunt resistance.

    Have a great weekend,

    Ian Williams