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ISO122 / ISO124

Mike H Meyer
Expert 1560 points

Other Parts Discussed in Thread: ISO122, ISO124

Are there any differences between the ISO122 and ISO124 other than nonlinearity gain?  The book price for the ISO124 is lower even though the nonlinearity gain is better so I'm trying to understand if there is anything else I'm missing.

Also, how does the signal propagate through if the power supplies are different on each side?  Is there a gain or could the signal simply hit the rail?

For my design +/-15V is available at the input so that’s easy, but only 0 to 9V at the output. I don’t understand how its referenced. Let’s say I have a 1Vpeak sine signal riding on 2.5VDC on the input, so it goes between 1.5 and 3.5V. When that gets output from the ISO124 with the 0 to 9V supply, does it try to match the center of the supply rails (treating it as a +/-4.5V supply) and can preserve this signal? Or maybe it sees the input signal as centered 17.5V above the negative supply such that the whole signal is clipped?  I just want to understand what to expect at the output.

  • 0
    TI__Guru* 93105 points

    Hi Mike,

    I use to support the ISO122 and ISO124 a few years ago so I hope I can help. See my answers along with your questions.

    Are there any differences between the ISO122 and ISO124 other than nonlinearity gain?  The book price for the ISO124 is lower even though the nonlinearity gain is better so I'm trying to understand if there is anything else I'm missing. The only difference I recall between them is the gain non-linearity limits being different. I don't think you are missing anything in the specifications.

     Also, how does the signal propagate through if the power supplies are different on each side?  Is there a gain or could the signal simply hit the rail? You can use different supply levels for the input stage and output stage providing the input and output voltage ranges are observed. They are galvanically isolated (they are not electrically connected) so they can be operated with different power supply levels. However, if you were to use +/-15 V to power the input stage and +/-5 V to power the output stage care would have to be taken such that whatever input is applied to the input stage doesn't cause the output stage to be driven into its supply rails. 

    For my design +/-15V is available at the input so that’s easy, but only 0 to 9V at the output. I don’t understand how its referenced. Let’s say I have a 1Vpeak sine signal riding on 2.5VDC on the input, so it goes between 1.5 and 3.5V. When that gets output from the ISO124 with the 0 to 9V supply, does it try to match the center of the supply rails (treating it as a +/-4.5V supply) and can preserve this signal? Or maybe it sees the input signal as centered 17.5V above the negative supply such that the whole signal is clipped?  I just want to understand what to expect at the output. The ISO122 and ISO124 are not specified for single-supply use. You can't directly power the output stage with a single +9 V supply. And remember the dc power supplies used for the input stage and output stage can't be the same supply or the galvanic isolation will be lost. 

    There have been some circuit where the input stage of the ISO122/I24 has been set up to use a single supply, but I haven't seen anything similar for the output stage. See Application Bulletin AB-009A:

    The best way to set up the ISO122/124 supplies is to use isolated dc-dc converters as described in Applications Bulletin AB-024. The components may be obsolete, but the bulletin portrays the method. I couldn't find it on the TI website, but did find it here:

    It does seem that a rail-splitter arrangement could be devised where the 0 to +9 V could be split to 0 V, +4.5 V and +9 V. Then , the GND of the output stage would be connected to +4.5 V, and the V- would be connected 0 V. I haven't ever done this so it is uncharted territory!

    Regards, Thomas

    PA - Linear Applications Engineering