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INA180: Current sense amplifier filter bandwidth

Wes J
Prodigy 10 points
Part Number: INA180


Tool/software:

I'm trying to read current of a motor driven with PWM using an H bridge (20kHz). I am using low side current sensing with a shunt and a TI INA180 amplifier. This will then feed the ADC of a microcontroller.
The amplifier output would be roughly a 20kHz square wave.
I found this reference design: https://www.ti.com/tool/TIDA-010056
The reference input filter bandwidth is ~2.4MHz, the output filter bandwidth is ~1.6MHz. But the amplifier they use only has a bandwidth of 150kHz. I'm assuming TI knows a lot more about this than I do, but I was under the impression that the amplifier had to be faster than the filter or things would become unstable. Can someone help me understand this?
Also, the INA180 has a max output of 8mA. How is it capable of driving a 1nF cap through a 100ohm resistor?
  • 0
    TI__Expert 7650 points

    Hi Wes,

    Wes J said:
    The reference input filter bandwidth is ~2.4MHz, the output filter bandwidth is ~1.6MHz. But the amplifier they use only has a bandwidth of 150kHz. I'm assuming TI knows a lot more about this than I do, but I was under the impression that the amplifier had to be faster than the filter or things would become unstable. Can someone help me understand this?

    The input filter is used to remove any high frequencies that may not be able to couple through from input to output, leading to adverse effects on the gain and output. The input filter (particularly the differential capacitor) also helps to mitigate any fast transients that may lead to a spike in the output. The output filter is to remove any residual high frequency noise that may have coupled in to the signal. 

    Wes J said:
    Also, the INA180 has a max output of 8mA. How is it capable of driving a 1nF cap through a 100ohm resistor?

    The 8mA applies to a DC current, the filters are significant for AC current and in general the INA180 can drive a maximum cap load of 1nF without oscillation as stated in the DS. When the capacitance is less than 1nF, use the output impedance vs frequency graph to select a resistor to place in series so that there's no sustained oscillation.

    Best,

    Mallika Senthil

  • 0 in reply to Mallika Senthil
    Prodigy 10 points

    I guess my question is:

    Is there a simplified way to design the output RC filter to avoid oscillations and still do its intended job?

    All of the anti-aliasing and charge bucket filter design guides are for regular op amps. There does not seem to be any guide for these black box current sense amplifiers. We don't know the gain band width, small signal bandwidth, large signal bandwidth, or settling time.

    So, what are the output RC filter bandwidth limits for this device and how does one determine that from the datasheet?

  • 0 in reply to Wes J
    TI__Expert 7650 points

    Hi Wes,

    I'm looking into this, I'll get back to you soon.

    Best,
    Mallika Senthil

  • 0 in reply to Wes J
    TI__Expert 7650 points

    Hi Wes,

    Please see the following app note, it maybe of help.
    https://www.ti.com/lit/an/sboa443/sboa443.pdf

    Best,
    Mallika Senthil

  • 0 in reply to Mallika Senthil
    Prodigy 10 points

    Yes, I have studied this app. It says:

     The assumptions made for the following analysis are:
    • The Current Sense Amplifier is 4 times faster than the filter

    If you apply the equations to the reference design, R23 should be ~15k ohms, C23 should be ~100pF, giving a filter bandwidth of ~100kHz.

    I still don't know how the reference design output filter values were determined. 

    Was 1nF chosen because the datasheet lists that as the max capacitive load?

    Was 100 ohms chosen to match the ADC impedance?

    Does the bandwidth even matter?

    The datasheet has very specific instructions for input filtering, but says nothing about output filtering.



  • 0 in reply to Wes J
    TI__Expert 7650 points

    Hi Wes,

    The app note discusses filters in terms of driving a SAR ADC, but the I believe the reference design has input and output filters to filter out high frequency components of AC CMRR, not for the purpose of driving an ADC, hence the amplifier speed isn't as critical.

    Wes J said:
    I still don't know how the reference design output filter values were determined.

    Since my team didn't make this design I can only draw an educated conclusion, I think the output capacitive value was determined from the max capacitive load (1nF) and the resistor was maybe set to match the ADC impedance.

    Wes J said:
    Does the bandwidth even matter

    That depends on the sampling frequency of the ADC.

    Generally, if the current sense amplifier output is to drive an ADC, the rule of thumb is that the output filter capacitor (Cpext) is 20x larger than the input capacitor (Ci) in the ADC. If its not used to drive an ADC, then for filtering purposes, select the resistor and capacitor values depending the frequency of components you'd want to filter out.

    https://www.ti.com/lit/ug/slau445i/slau445i.pdf?ts=1750456275306


    Best,
    Mallika Senthil