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INA168-Q1: INA168 - CURRNE SENSE PROBLEM

IGAL TALOR
Prodigy 20 points
Part Number: INA168-Q1
Other Parts Discussed in Thread: INA168, INA186

Hi All,

I am using the component INA168:

RS = 0.01 OHM

RL = 133KOHM with 100pF parallel capacitor

after it i have buffer amplifier AD8615 with RC output R=10ohm C=1nF and this is connected to ADC

I need to measure current from 0 to 15 amp

when the load current is 1.7A - the RS voltage 17mV

at the output I measured 350mV and the voltage should be 450mV according to data sheet

I don't know why, I didn't sleep all night because of thinking about it.

please help.

  • 0
    Prodigy 20 points

    the load voltage = 24V

    INA168 voltage = 5V

  • 0
    TI__Mastermind 31915 points

    Hello Appreciated Engineer,

    I am looking this over and will respond shortly.

    Sincerely,

    Peter

  • 0 in reply to Peter Iliya
    TI__Mastermind 31915 points

    Hello again,

    By load voltage I assume you mean the input voltage at INA168 input pins?

    And by "INA168 voltage" you mean the INA168 supply voltage at V+?

    Would you try confirming the accuracy of your current setting? What is setting the 1.7A load? 

    What is the tolerance of the Rs and RL?

    Would you please send a basic image of the Rshunt layout?

    Also a layout or diagram of how INA168 ground connects to 24V source return would be helpful.

    What happens to accuracy when you measure higher currents? Please provide a few data points.

    Sincerely,

    Peter

  • 0 in reply to Peter Iliya
    Prodigy 20 points

    Load voltage I mean Vin+ Vin-

    INA168 voltage I mean supply V+

    the load now is a load resistors, I have a power supply with current monitor and I used FLUKE to measure the current.

    The tolerance of Rs and Rl are 1%

    When the current 1.7A the value of INA186 is 1.3A

    When the current is 4.5A the value of INA186 is 3.8A

  • 0 in reply to IGAL TALOR
    Prodigy 20 points

    I measured on input pins Vin+ and Vin- 17mV

  • 0 in reply to IGAL TALOR
    TI__Mastermind 31915 points

    Hey Igal,

    1. How are all IC and instrument grounds connected?

    2. How are you measuring INA168 output voltage? With ADC or with a voltmeter? When using a voltmeter, please probe with respect to INA168 ground pin (as close as possible).

    3. How did you measure the differential input voltage (VIN+ to VIN-)?

    4. I recommend ensuring all ICs and instruments have their returns/grounds connected back to one single ground point, usually the 24V source return, and with low-impedance cables.

    5. With external voltmeter measure the difference between INA168 ground pin and other ground points in your circuit.

    6. Try simplifying the circuit and disconnect INA168 from op amp. 

    Sincerely,

    Peter

  • 0 in reply to Peter Iliya
    Prodigy 20 points

    1. below all  the component I have ground and supply planes.

    2. the measurement with ADC, when I measuring with FLUKE the probes are on the output capacitors on both side.

    3. I measured with FLUKE 87 with mv scale.

    I have in my circuit 20 more INA168 and the are measuring good - but there I am with Rs=0.02

    maybe Rs=0.01 is very low?

  • 0 in reply to IGAL TALOR
    TI__Mastermind 31915 points

    Hey Igal,

    The lower Rs will affect accuracy, but only at smaller currents due to input offset voltage. This error should become negligible at higher currents. Error at 15A load would be helpful to know.

    Would you please provide a schematic? One of the 10mΩ circuit and one of the 20mΩ circuit for comparison? Please include the whole signal chain up to ADC.

    Please also send screenshots of the shunt layout as well (one for the 10mΩ and another for the 20mΩ circuit).

    If you do not want to send this information here, you can message me privately or through my email.

    Sincerely,

    Peter

  • 0 in reply to Peter Iliya
    Prodigy 20 points

    I sent you a private message

  • 0 in reply to IGAL TALOR
    TI__Mastermind 31915 points

    Hey Igal,

    I have responded back in the private message.

  • 0 in reply to Peter Iliya
    TI__Mastermind 31915 points

    Hey Igal,

    Since I have not head back from you, I will mark this thread as resolved, but if you have any updates, questions, or concerns you can simply reply back or start a new thread.

    Sincerely,

    Peter