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Original question:

INA240: INA240

INA240: Output error estimation

Daisuke Higa
Genius 4535 points
Part Number: INA240


Tool/software:

Hello support team all.

I am attempting to measure bidirectional coil current in a circuit using an INA240A1 to control the coil current in an H-bridge circuit.
The applied voltage is 48V. The MOSFET Ron value is 20mΩ, and the maximum current is 6.25A. (See the diagram below.)
I would like to predict the output voltage of the INA240A1.

I understand that the output voltage error for the INA240A1 is calculated according to the formula in the table in 8.4.4.1 Error Sources in the datasheet.

The calculated error results are shown in the attached Excel spreadsheet.
Could you please check that these results are correct?
If the results are correct, I can assume that a large offset occurs when the common-mode voltage is low (off period), causing waveform distortion.
Is my understanding correct?

Error estimation.xlsx

Best regards,

DH

  • 0
    TI__Mastermind 33320 points

    Hello DH,

    Daisuke Higa said:
    Could you please check that these results are correct?

    The calculations are correct but the CMRR is set for 50kHz.  This will not be the case as the transition form 0V to 48V could be much higher.  Maybe you will filter out the higher frequencies.  The calculations I see are also done with typical values and that is fine just be aware you could see some values larger than that.

    Daisuke Higa said:
    If the results are correct, I can assume that a large offset occurs when the common-mode voltage is low (off period), causing waveform distortion.
    Is my understanding correct?

    This will be the case as you can see and example in the datasheet in figure 9.4 attached below for convenience.

    Please keep in mind that using the Ron of a FET will change and not consistent.  This will add to your error source.  The other thing is that if you have an external resistor please pick on with low inductance as di/dt will cause a larger voltage across the resistor than expected.

    Regards,

    Javier

  • 0 in reply to Javier Contreras4
    Genius 4535 points

    Hi Javier-san

    Thank you for your advise.

    Are there any useful resources available regarding high-frequency filtering?

    BR

  • 0 in reply to Daisuke Higa
    TI__Mastermind 33320 points

    Hello DH,

    You can add some RC filtering at the input but it is recommended to keep the Resistance under 10Ω due to the IB of the INA240.  The issue is this filtering needs to be common mode for the device while still getting the Vsense voltage at high speed.  The mismatch of resistors and capacitors could cause even more issues at higher frequency.

    The INA241 is a little better for the VCM transition but approaches the problem in another method.  Datasheet is best resource for this.  

    I would place a resistance you show in your schematic and not use the Ron of the MOSFET.  I would then pick between the INA240, INA241 or INA296.  The issue with the INA241 is the time between pulses and when they get near the 3µs.  This section in the INA241 datasheet covers it (7.3.1.1 Enhanced PWM Rejection Operation).  https://www.ti.com/lit/ds/symlink/ina241a.pdf#page=15

    Regards,  

    Javier