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Op Amp specs for motor phase current sensing

Lucas Schulte9
Expert 6955 points
Other Parts Discussed in Thread: INA181, OPA607

So I'm trying to pick out the proper dual channel SOIC 8 op amp to use as a current sense amplifier. I understand how to set up the op amp to achieve this but I have questions concerning the properties of the op amp.

1. How do I determine what bandwidth is needed? The control loop speed of my application is about 30kHz. Switching frequency can be up to 40kHz. The gain for the amplifier will be 20 so I know this reduces the BW of the op amount by a division of 20. Does this mean the op amp should be at least 30kHz x 20?

2. How do I determine the slew rate? I see this specification often as one of the headlining properties of an op amp. I know what slew rate is but I'm not sure of its significance to my application.

  • 0
    Guru 140200 points

    Hi Lucas,

    this is difficult to answer without seeing any schematic or scope plots. If your signal bandwidth is 30kHz, you should have a gain reserve (or loop gain) of least 40dB at 30kHz and for a chosen closed loop gain of 26dB. Then the OPAmp is able to regulate the error to under 1% with its feedback loop. This means an open loop gain of 26dB + 40dB = 66dB at 30kHz which translates to a bandwidth of OPAmp of about 60MHz, as a rule of thumb.

    Slew rate can be estimated by this: The OPAmp should be able to settle to the final value within a period of let's say 1% of period of maximum frequency which translates to about 300ns settling time and slew rate of about 30V/µs for a 0V to 10V step.

    But if your signal bandwidth is considerably higher, which I guess, then you might even need a faster OPAmp.

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 6955 points

    Kai,

    Thanks for the response. Surely though the op amp wouldn't need that high of specs. I've successfully used the INA181A1 for this exact purpose and it doesn't have specs anywhere near what you have mentioned. The reason I don't want to use it is because I want a flexible footprint/socket. So I guess really all I need is an op amp that would perform with same specs as the INA181A1 after it is set up at a gain of 20 for bidirectional current sensing.

    Respectfully,

    Lucas

  • 0 in reply to Lucas Schulte9
    Guru 140200 points

    Hi Lucas,

    why haven't you mentioned in your original post that you look for an alternative to the INA181?

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 27715 points

    Hi Lucas,

    I am re-assigning your thread to the precision amplifier team so they can help you with an alternative to the INA181.

    Best regards,

    Ian Williams
    Applications Manager
    Current Sensing

  • 0
    TI__Expert 3126 points

    Hello Lucas, 

    I believe the OPA607 shall be the perfect part for this application , 

    The OPA607 application section in the datasheet discusses the calculation of the required bandwidth for current sensing in motor drive current sensing application it also takes into account the additional BW needed for  the responsibility of driving the subsequent ADC as well .

    http://www.ti.com/lit/ds/symlink/opa607.pdf

    OPA607 is a 50MHz , 24V/uSec slew rate opamp with just 1.5uV/C of offset drift making it the ideal part for current sensing .