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INA181: AC current sensing and Resolution

Elisha Tarroza71
Expert 3700 points
Part Number: INA181

Hello,

1. According to this blog, shunt can measure both AC & DC current. Since INA181 is a "current-shunt monitor" according to the datasheet, does it follow that the INA181 can measure both AC & DC currents?

2. Is there a way to know the minimum current can be measured (or resolution) and what will be the voltage for that minimum current?

Thank you!

  • 0
    TI__Mastermind 26750 points

    Hey Elisha,

    1.The INA181 can measure AC and DC current as long as the input shunt voltage is within its full-scale range. It can also measure positive and negative currents as long as its output is offset to some voltage (usually Vs/2) so the negative currents won't saturate the output. Consider watching out Precision Labs training video for current sensing here:

    https://training.ti.com/ti-precision-labs-current-sense-amplifiers-gain-error?context=1139747-1139745-1138708-1139852-1140045

    2. The minimum current and resolution will be different and it all depends on the shunt resistor, device gain, supply voltage, and reference voltage. These values will be set by knowing what the current range to be sensed is, what is the max error allowable at the minimum sense current. The above link can also help out with these concepts.

    To further answer your question, the resolution will be bounded by the output pin's peak-peak noise. Output noise is calculated the exact same as when determining noise for operation amplifiers discussed here:

    https://training.ti.com/ti-precision-labs-op-amps-noise-1?context=1139747-1139745-14685-1138803-13232

    So you can improve resolution by filtering the output signal. So the total peak-peak output noise for INA181 is calculating as follows:

    BWn = noise bandwidth = Kn*BW = 1.57*BW;   

    BW = device's closed-loop bandwidth

    Kn = brick wall correction factor for different filter order.

    En_shunt = SQRT(4*k*T*R*BWn)

    En_csa = eBB*SQRT(BWn)

    eBB = device's input spectral noise density (nV/rtHz)

    En_total_rms = E_shunt^2 + E_CSA^2

    En_total_rms_RTO (referred to output) = En_total_rms*GAIN

    En_total_peak-peak = 6*E_total_rms_RTO

    Hope this helps,

    Peter