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INA138: its under-amplifying or I'm doing something wrong?

Weiwei Su
Prodigy 250 points
Part Number: INA138
Other Parts Discussed in Thread: INA190, INA186

Tool/software:

Hi team,

I'm building a motor driver board where I used a INA138 to measure the supply current, because the board would consume nearly 320A at peak, so I've used 2 0.25mR resistor in parallel can connects it in a kelvin sensing fashion.

but the output is almost unuseable. Per the calculation it should have readings for around 1A, but it only starts producing output after around 5A. I almost thought the chip is broken.

and it has lots of noises too. Not sure if it came from the power supply and it just amplifed it as well. BLDC motor is indeed very noisy.

should I max out the gain RL resistor or pick a higher resistance current sense resistor?

  • +1
    TI__Mastermind 32670 points

    Hello Weiwei,

    The input offset voltage is 1mV which is about 8A using your Rshunt of 125µΩ which is what is most likely your limitations at the output.  The other issue I see is your bandwidth is very low with the 250kΩ and 0.1µF capacitor at the output.  Changing the output resistor will not help in this situation as this is input error that we are dealing with. Increasing your Rshunt will help with this.  We have other devices that have lower input offset voltage <100µV

    Please also keep in mind the the transconductance is only specified with input from 10mV to 150mV inputs.  It should still function below but the transconductance may vary more than what is specified.

    Regards,

    Javier

  • 0 in reply to Javier Contreras4
    Prodigy 250 points

    Mr. Contreras,

    Increase Rshunt might facing trouble with the current capacity, while I will try that for the moment, can you recommend me another devices you were stating with lower input offset?

    Also you said that the bandwidth is low, what's the recommended combination for this specific application?

    Thanks

  • 0 in reply to Weiwei Su
    TI__Mastermind 32670 points

    Hello Weiwei,

    I can make recommendations but I would need to know key specification to limit the search.  Here is some limits with under 100µV of error.  Cost changes with a couple of parameters such as bandwidth and VCM range so that is normally what I start my narrowing parameters with.  https://www.ti.com/product-category/amplifiers/current-sense/analog/products.html#773=50&766max=8%3B100&sort=773;asc&

    If accuracy is biggest concern I would say the INA190 lower cost lower accuracy version is the INA186.  Any other information will help in getting the best device.

    We have a calculator here so you can pick between our devices. https://www.ti.com/tool/CS-AMPLIFIER-ERROR-TOOL

    Please take a look at specification on offset and swing low for the minimum output voltage.

    VCM range(BATT voltage range):
    Bandwidth Required:

    Bandwidth requirement is understanding how it is being measured and how fast it needs to get updated.  Here is an example if driving a SAR ADC.

    https://www.ti.com/lit/an/sbaa081a/sbaa081a.pdf

    Regards,

    Javier