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AMC1301: The bandwidth and the measured current frequency

Steve Sun
Intellectual 1890 points
Part Number: AMC1301
Other Parts Discussed in Thread: AMC1300

Hi, I have a question when I design a current measurement circuit. For an ac current with a certain frequency (e.g. 20kHz), how to select the bandwidth of the amplifier to use? Moreover, for shunt-resistor-based scheme, the bandwidth of the whole measurement circuit is just the bandwidth of the amplifier? or it is related to the shunt resistor? 

  • 0
    TI__Guru 51652 points

    Hi Steve,

    Is this for an isolated measurement or a non-isolated? 

    In general, you want the bandwidth to be at least 2x the frequency of the signal to be measured to satisfy nyquist requirements, additional bandwidth is preferable as high frequency components may be attenuated. 

    For extremely high frequency applications and the shape of the signal to be measured, the bandwidth of the shunt resistor (mainly the inductance) comes into play. Larger package shunt resistors with low inductance can be used. 

  • 0 in reply to Alexander Smith
    TI__Intellectual 1890 points

    Hi, this is for an isolated measurement. Is there any difference between isolated measurement and non-isolated measurement when selecting bandwidth of the amplifier? 

    Larger package shunt resistors with low inductance are selected because of good heat dissipation and large bandwidth?

    By the way, how to determine the current range that an isolated op amp can measure？

  • 0 in reply to Steve Sun
    TI__Guru 51652 points

    Hi Steve,

    With isolated amplifiers, the bandwidth is fixed, typically around 300kHz for our current generation devices due to the integrated digital filter. Non-isolated amplifiers like OPA's or INA's bandwidth will be set by device or passives in the feedback loop, but there is more flexibility. 

    Larger package shunt resistors due typically have higher power dissipation levels and their low inductance will create a smaller "offset" due to changes in current polarity (di/dt). 

    Our current sensing isolated amplifiers currently have +/-50mV or +/-250mV input voltage ranges. In general, smaller input voltage ranges are better for measuring higher currents as the resistance of the shunt resistor will be smaller and therefor less power dissipation (R*I^2) from the shunt. 

    I've created an excel calculator for assistance with sizing the shunt resistor: https://www.ti.com/lit/zip/sbar020

    as well as a paper to assist with selection: https://www.ti.com/lit/an/sbaa460/sbaa460.pdf?ts=1684179841117

    Here is also a link to other general collaterals that may help: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1164993/faq-isolated-data-converter-collateral-overview

  • 0 in reply to Alexander Smith
    TI__Intellectual 1890 points

    Hi, thank you very much for the detailed answer and sharing. Finally, I have another question, I have not found any parameters (e.g. output current) in the AMC1301 datasheet that can describe its output capability.

  • +1 in reply to Steve Sun
    TI__Guru 51652 points

    Hi Steve,

    There are some parameters related to the output of the isolated amplifier, but it's mostly accuracy related as the output is set by the digital filter. These devices are meant to be interfaced with another OPA for differential to single-ended conversion or connection to a differential ADC. There are cookbook examples in the ppt I linked earlier showing this. Output current would be a parameter for a non-isolated amplifier, the designs are very different. 

    Also: please promote the AMC1300 in place of the AMC1301, it's p2p compatible and built on a newer process.