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[FAQ] Can a digital isolator signal voltage be different from its power supply voltage?

Calvin Chan86
Prodigy 380 points

Other Parts Discussed in Thread: ISO6741, ISO6441

Can a digital isolator signal voltage be different from its power supply voltage?

  • 0
    TI__Prodigy 380 points

    In datasheets of digital isolator, such as ISO6741, the absolute maximum input voltage is VCC+0.5V. This means that the input voltage is dependent on the VCC supply voltage. Crossing this specification violates the absolute maximum ratings and can cause damage to the device. For these devices, it is recommended to keep signal voltages similar to the isolator power supply voltage. For example, when ISO67xx is powered with 5V and is interfaced to an MCU it is important that the MCU signals are also operating at 5V logic levels.

    In datasheets of digital isolators with overvoltage tolerant inputs, such as the ISO6441, the absolute maximum input voltage is -0.5V to 6V (shown below). Unlike the ISO6741, the input voltage is a numerical value and not dependent of the supply voltage. This means that the input signal can be applied even when supply voltage is powered off, as long as the applied signal is within absolute maximum ratings. For example, when ISO64xx is powered with 3.3V and is interfaced to an MCU, the isolator can still have a 5V signal applied.