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ISO124: Can be use for line signal sensig?

Gabriel Barrionuevo
Prodigy 30 points
Part Number: ISO124
Other Parts Discussed in Thread: ISO224, INA129

Hi! I am working on a project in which I need to measure the household line voltage (220V). I know there are devices similar to ISO124 specifically designed to perform these measurements and provide insulation. But in my country (Argentina) I can only get the ISO124 so I try to use this device for my requirement.

Basically I intend to adapt the line voltage with a voltage divider to manageable 5V peak to peak or less, and then place ISO124 to provide isolation to the rest of the circuit. Then an instrumentation amplifier with the only purpose of entering a reference voltage of 2.5V so that the signal always has positive values. Then, the output signal goes to the A / D converter of my arduino. But I have a great doubt regarding the connections of ISO124 since I have not been able to resolve this from the data sheet:

I do not understand if on the GND1 pin I should only connect the power supply ground, or if only the neutral line cable would go in this case, or if in that case I should connect both, that is the neutral cable and The ground of the symetric power supply. As I read that the device does not perform a differential measurement, I assumed that the GND1 pin should have the neutral line cable connected to it and reference the same ground pin of the power supply to have the positive and negative voltages from the symetric power supply at + Vs1 and - Vs2 respectively. Following the same logic is that on the side of the output I connect the ground of the second power supply to the GND pin. I leave a picture of my design so you can take a look and tell me if my reasoning and design is correct or if I should change something. I have indicated with a box in blue dotted lines in the areas of interest. Of course, I appreciate your time, and I hope you can help me.

  • 0
    TI__Guru 50541 points

    Hi Gabriel,

    By chance, are you able to get the ISO224 in Argentina? 

    I only ask because it is the newer iteration of the ISO124. That being said, I do not see an issue with using the ISO124 for this application. 

    You would want to connect both the power supply and neutral cable to GND1. The low side power supply GND should be connected to GND2. 

    From looking at your schematic and the INA129 datasheet, I think you are correct, however I am not an expert with the INA129. You may want to make a another post to ask that team what they think. 

    This app note may be useful: http://www.ti.com/lit/an/sbaa317/sbaa317.pdf

    I think the INA129 may be overkill for this application. You can probably use a simpler opamp like in the note above for level shifting. 

  • 0 in reply to Alexander Smith
    Prodigy 30 points 
    Hi Alex! Thank you very much for your answer! Let's see...
Unfortunately I cannot get ISO224 here, I have looked for it but I had no luck. I know that it is the IC that best suits my requirements and I had already read before those notes that you shared among others that I also read, and that is how I concluded in the scheme I designed with ISO124 since it is the only one I get.
On the other hand, you are right, maybe the INA129 is too much for my application, I will review that and maybe change it for a simpler opamp although it could work as it is. But this is not the main thread of the post so I could consult that in the appropriate forum.
My main question was in the connection of pin GND1. Two options revolved through my head:

1.- Only connect the line neutral to the GND1 pin.
2.-Connect the line neutral and the power supply ground to pin GND1.

So, to close this post and make everything clear:
The correct option is 2? and therefore my connection scheme is correct and is it possible to use ISO124 for this application?

Again, thank you very much for your time. Regards!
  • 0 in reply to Gabriel Barrionuevo
    TI__Guru 50541 points

    Hi Gabriel,

    Option 2 is correct and it is possible to use ISO124 for this application. 

    Happy to help! Feel free to create a new post if you have more questions. 

  • 0 in reply to Alexander Smith
    Prodigy 30 points 
    Thank you very much for your help, Alex. I will continue with my project and if I find new questions, I will be here again.
Regards!