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FRANK1
Expert 8675 points
Other Parts Discussed in Thread: OPA1671

Greetings,

I want to select a TI Op amplifier or a suitable IC back-end of LEM Current Transducer
www.lem.com/.../has_50__600-s.pdf

it will can convert the AC voltage ±4V from LEM Current Transducer to DC 0~4V linearly.
high speed, and Step response <3ms.

is there any suggestion on this?

Thank u so much.

Regards,

  • 0
    Guru 140200 points

    Hi,

    how shall the OPAmp be powered? Dual supply voltage? Single supply? Do you want to drive an ADC? How is this ADC powered?

    Kai

  • 0
    Intellectual 420 points

    You can use nearly any OpAmp, it seems.  The LEM uses +/-15V so if you use those rails, look for a 36V-rated OpAmp.  The offset from the LEM is +/-20mV so offset is unimportant.  The response of the LEM is 50kHz where a step response of 3us might be available.  There are few amplifiers with responses this low.  No idea what kind of noise you need.

    Audio amplifiers are a good place to look.  An OPA1671 is available in a single device available in SOT23 or SC70, for instance.  There are many factors that can make one device better or worse in your application.

    If you don't know what a difference amplifier is, do a quick search.  4 resistors and you can take an arbitrary input range and offset to an output range and offset.

    Happy designs!

  • 0
    TI__Mastermind 32395 points

    Hello m123456,

    Perhaps you could half-wave or full-wave rectify the +-4V AC output and then low-pass filter this to average and convert it into 0-4V output. See pages 122 and 126 for the rectification circuits in our analog circuit cookbook: https://www.ti.com/seclit/sl/slyy137a/slyy137a.pdf?ts=1591312852949

    There are also low-pass filter circuits in there that could be useful too.

    I believe you will have to throw away bandwidth in order to convert an AC voltage into a DC range so knowing this requirement will be necessary. It seems you want a response time under 3ms. This will bound the low pass filter to RC = 1/3ms = 333 Hz. You will want to set your cutoff frequency to something higher depending on how accurate your settling value needs to be. For a 6-sigma settling value, this equates to 6*333.33Hz = 2kHz. 

    I highly recommend using our simulation models to play around with the low-pass filtering and gain needed to make to conversion (https://www.ti.com/design-resources/design-tools-simulation/models-simulators/spice-library.html)

    Overall, the op amp will need a positive and negative power supply of > 8V differential supply at best. Probably will want to power the op amp with a +/-8V supplies depending on what their operation VCM range is.

    Sincerely,

    Peter