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common mode rejection vs common mode voltage

Discussion in 'Electronic Design' started by Jamie Morken, Feb 23, 2008.

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  1. Jamie Morken

    Jamie Morken Guest


    I was looking at the datasheet of the HCPL-2201, and it specifies
    a minimum CMR value of 10kV/us at a common mode voltage of 300Volts.

    When the common mode voltage is less, ie. 50volts, the CMR goes down
    to something around 1kV/us.

    I have a fet driver circuit that requires a minimum of 10kV/us CMR
    for the opto, for a floating fet gate, would this opto work properly
    under these conditions? I am not sure how to know what the common
    mode voltage would be to know if I will get 1kV/us or 10kV/us CMR
    or something in between.

    HCPL-2201 datasheet:

  2. legg

    legg Guest

    You're misreading the spec.

    On page 12:

    HCPL2211/2 has the higher CMRR
    HCPL2201 has the lower CMRR.

    For a specific part number, the CMRR varies, at a specific forward
    current, depending upon whether the CM is applied when output logic is
    high or low.

    In one state, the applied opto current is effectively nulled by CM
    In the other logic state, all apparent opto reciever current is due to
    dvdt effects alone.

  3. Terry Given

    Terry Given Guest

    I have somewhere an app note that describes how to improve the CMR of
    optos. basically DONT drive the led open-ended, as when the LED is off,
    the impedance preventing any current flow is very high, so dV/dt can
    capacitively couple current thru to the LED side which can then flow
    thru the LED turning it on..... a pair of gates driving the LED in
    push-pull is a good solution; the other is a resistor to Vcc to turn it
    on, and shorting the LED to turn it off. hungrier this way though.

    ISTR some of the FOD series optos have fast enough prop. delays Jamie....

  4. Jamie Morken

    Jamie Morken Guest

    I think this is the way I have it set up (resistor to VCC and transistor
    to gnd):
    " fet driver.jpg"

    the two wires on the left lead to a transformer secondary (floating fet
    driver supply)
    Yes I've seen some of these, they are mostly 5V supply though, and would
    require a small 5V regulator for each floating fet driver, nicer to have
    a opto that can take up to 20V :)

  5. Jamie Morken

    Jamie Morken Guest

  6. Jamie Morken

    Jamie Morken Guest

    Here's the recommended LED drive for ultra high CMR:

    " CMR optodrive (HCPL-3120).jpg"

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