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Original question:

INA236: Current measurement change depending on conversion time

INA236: Current measurement change depending on conversion time and cap

Sébastien Lagacé
Prodigy 155 points
Part Number: INA236

Tool/software:

Hello TI,

I'm reopening this thread, i have new informations that i think will help pinpointing the issue.

RECAP of other thread : 

-Current measured when using only current measurement is very close to actual current.

-When adding the voltage measurement there is a large deviation on the measured current. The longer the sampling time, the closer we get to the actual current.

Setup :

We monitor the current of a high side switch 14V - 0-15A

Rsense = 0.5mohms

Rflt = 100R 

Cdiff = 1uF

Sampling time 332us (avg: 64)

Resistive load for 1.4A 

New information : 

-The filter seems to have a large impact, removing the 1uF solves the issue, a 0.1uF helps but is still too much.

-The error is an offset, at 15A or at 1A, it is around the same delta in mV

Any ideas why the capacitor impacts the measurement, i would like to understand the issue to choose the filtering capacitor and resistors that are the best suited for this application.à

Thank you.

  • 0
    TI__Mastermind 34180 points

    Hi,

    For the offset error in mV at 15A and 1A, what is the magnitude?

    The way the filter cap impacts accuracy is unusual. Are you comparing Vshunt with DMM measurements at the input pins?  

    Regards

    Guang

  • 0 in reply to Guang Zhou
    Prodigy 155 points

    Here are my measurements with 0.1uF i have an precise eletronic load and i use the same current load settings to compare with or without the voltage measurement.

    Only current measured 332us sample time

    2A - 2.04A

    5A - 5.12A

    10A - 10.23A

    14.5A - 14.85A

    Same thing with Voltage and current measurement (offset of around 16mV when adding the voltage measurement)

    2A - 2.2A

    5A - 5.27A

    10A - 10.4A

    14A - 14.5A

    it looks like the adc sampling cap does not have time to fully settle going from 12V range to 0.03V range with this acquisition time. I'm not sure how removing the cap changes that behavior. The related thread showed the magnitude of error with 1uF cap which was a lot worse.

    1.3A actual would measure 2.4A with AVG:64 sampling time: 332us (same setup as for all above measurement)

    Thank you.

  • 0 in reply to Sébastien Lagacé
    TI__Mastermind 34180 points

    Hi,

    If the cap is removed all together, doe the reading change again? Are there resistors on the input path?

    It might be a good idea to measure input voltage with a DMM to see which set of results are close. For Example, at 10A, the DMM should measure 5mV.

    Regards

    Guang

  • 0 in reply to Guang Zhou
    Prodigy 155 points

    Hi, I mentionned this in the first post.

    i use 100R resistors with the 0.1uF cap for filtering. Below is the full circuit. Removing the cap completely brings me even closer to the real value.

    I get closer to the real value by either reducing the cap or increasing the sample time. 

    I have not done the DMM measurement but we did enough measurements to know which measurement is correct.

    My hypothesis is that the sense resistor is too low. As you say 5mV at 10A is very small probably adding the voltage measurement induce a normally negligible error but with or measurement being so small it has a big impact. I'll increase the resistor value of the sense resistors and it should be fine.

    I have done a verification of the sense sizing previously but might have missed a zero in the sense value. you saying i should measure 5mV made me realize this value makes no sense.

     

    I'll create a new case if i still have an issue when i test the new value. Thank you.