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ISO7140CC: Use as Level Shifter

Gordon McKenzie
Intellectual 580 points
Part Number: ISO7140CC
Other Parts Discussed in Thread: ISO6740, ISO7740

Hopefully an easy question...

I'm looking for something that will do a level shift from a 0V/3V3 digital control signal to +/-2V5 (signals to control a split-supply amp, where they have to be referenced to -2V5 rather than 0V. The normal level translators aren't any use because they expect a common ground reference on both sides.

I think I can use the ISO7140, but it's not clear from the abs. max ratings whether or not I can have a voltage difference between the two grounds. I want to connect the supplies as follows:

GND1: 0V

VCC1: 3V3

GND2: -2V5

VCC2: +2V5

I can do it quite simply with a couple of FETs and resistors as per diagram below, but would like something that can switch a bit faster and 4 channels in one package would also help so a 4-channel isolator looks a good option. Is it OK to connect it like that?

  • +1
    TI__Expert 5335 points

    Hello,

    Thank you for your question. 

    From my understanding, you would like to convert your digital signal from one voltage to a different voltage, where the signal goes from 3.3V on one side to 2.5V with a difference in ground being -2.5V.

    Yes, using the ISO7140CC in this manner is fine.

    Our digital isolator devices can be supplied voltages within the recommended operating conditions on both sides of the device. Since the isolation barrier separates the two sides, each side can be sourced independently with any voltage within recommended operating conditions. For more information on having different voltages on either side of a digital isolator, please checkout this FAQ:[FAQ] Can I power the two sides of a digital isolator with different voltages?

    Our digital isolators can also tolerate ground potential differences up to several thousand volts. Since your difference is -2.5V, the ISO7140CC device should work for your application.

    I would like to recommend another device, the ISO6740. The ISO6740 is a reinforced isolator that supports up to 50Mbps. The ISO6740 device is a newer, lower cost, and more popular device that we currently offer. Since the ISO7140CC is an older device, we highly recommend the ISO6740: ISO6740 data sheet, product information and support | TI.com

    If you require a higher data-rate device for your application, we would recommend the ISO7740 device, which supports up to 100Mbps: ISO7740 data sheet, product information and support | TI.com

    Regards,
    Aaditya Vittal

  • 0 in reply to Aaditya Vittal
    Intellectual 580 points

    Thanks, Aaditya. It does seem quite safe to use in this way, as I expected it would be.

    I looked at the ISO6740 but I was put off a little by the very large package size, which I guess is a result of a large isolation barrier. If I can fit it in easily, I'll certainly consider it.

  • +1 in reply to Gordon McKenzie
    TI__Expert 5335 points

    Hi Gordon,

    you're welcome!

    Please feel free to let me know if you have any additional questions.

    One thing I would like to mention is that the ISO7740 device comes in smaller package sizes, which may work for your application: ISO7740DBQ

    Regards,
    Aaditya Vittal