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How to use the current sens amp for PWM motor application?

Hi

I would like to know how to use current sense amp in PWM motor drive system.

For example, AD8418 has a peak output by input common-mode step response.
AD8418A,AD8417, AD8210 .... also.
(see AD8418 datasheet Fig.18)

These noise are tough to erase by LPF.
I don't know how to erase these noise.

One idea is to use the differential signal from 2 phase current sense amp outputs.
I don't know if it is good or not. and it is hard to use.

Could I see the recommended circuit?

Regards
moto


  • Hi, moto.

    The output peaking in the common-mode step response graph is always visible in all current sense amplifiers because this is the reaction of the inputs of the current sense amplifier for every common-mode voltage that it receives. The higher the common-mode voltage, the higher the peaking but this is only in mV range.

    An output filter (RC, LC, or amplifier) can be used to filter this out but the more important spec that you must consider in PWM system using current sense amplifier is the settling time of common-mode step response. This should be in nanosecond range and AD8418 is within this range.

    Regards,

    Anna

  • Dear Anna

    Thank you very much for your explanation.

    nano sec peaks will be OK, but actually it was about 1us.(it might be depend on the system, I think)

    so it was tough to eliminate.

    This output peaks does not depend on the sensing current value, I think.

    The high shunt resister will make better SN?

    Regards

    moto

  • Hi, moto.

    Using a high value of shunt resistor will disrupt the flow of current in your circuit in such a way that the resistor will receive a very small amount of current thus producing a low input voltage. Better use a shunt resistor that will receive your desired current. Also, high resistance yields high noise so use a very low resistance precision resistor with at least a tolerance of 1% or 0.5%.

    Regards,

    Anna

  • Dear Anna

    Thank you for your support.

    You are correct.

    but the noise with small shunt resistor means high error signal.

    I understood this product requires good LPF design.

    In Datasheet Rev0 P13 Figure33,

    REJECTION OF HIGH PWM COMMON-MODE VOLTAGE(-2V TO +70V)

    We don't have a reference circuit, so we should be design to eliminate these peaks in the system.

    I got.

    Regards

    moto