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DC line isolation using ISO124 for ADC measurement

Murat Celebi
Prodigy 50 points
Other Parts Discussed in Thread: ISO124, ISO120

Dear Forum Community,

I am planning to use ISO-124 isolation amplifier for DC line isolation with 1V/V gain. But I have two supply voltage to ISO124 that input voltage is 28V DC and output supply voltage is 12V DC. But Our  voltage sources supply unipolar voltage.  I'm not sure exactly, ISO-124 are working with Bipolar voltage supply.  But I don't convert voltage sources from Unipolar to Bipolar type using external components. Also  I am thinking to use 28V DC for  Vs1  and 12V DC  for Vs2.

At this configuration, I want to monitor input supply voltage(28V) . I am using voltage divider for input voltage monitoring. Input voltage's GND and ADC's GND are isolated by DC-DC converter. So I want to use ISO-124 for monitoring signal from Voltage Divider to ADC. Measurement accuracy is not very important to me. Tolerant can be high value. But Gain that between input and output must be 1 V/V.  Could you please help me this problem solution? Could you suggest reference design for our design? My reference desing is showing in the picture below.

  • 0
    Prodigy 50 points
    Does no one to help me? Dc line isolation circuit is important for me? If needed circuit don't run desired, Could you please suggest another device or solution for my design?
  • 0 in reply to Murat Celebi
    TI__Genius 9975 points

    Hello Murat Celebi,

    Please remember that you need to apply bipolar supplied to the ISO124. The 28V and 12 V supplies you are considering will not allow the ISO124 to work properly.

    The second point to remember is that the ISO124 swing to the rail is ensured only up to 5 V from the rail; so, if your minimum supply voltage on +VS2 and -VS2 is +/- 6V, then you can only measure signals that are up to 1 V above ground and down to 1 V below ground.

    Page 16 of the ISO124 datasheet shows one example of an application where the VS1 side is supplied with a unipolar supply. Please consider something like that for your application.

    Best regard,

    Jose

  • 0 in reply to Jose Duenas
    Prodigy 50 points

    Hello Jose;

    Thanks to reply. I will consider the points that you say. My minimum supply voltage on +VS2 and -VS2 is +/- 6V, then I want to measure signals  that are up to 1 V above ground and down to 1 V below ground for Vout. Because I will apply signals to Vin  that  are up to 1 V above ground and down to 1 V below ground.  So Gain will be about 1 V/V.  I saw, Vin  and Vout  is changing linear up to 7V  when I run  below simulation circuit for ISO120  on multisim.   According to you, Can I use this  design  for my circuit?

  • 0 in reply to Murat Celebi
    TI__Genius 9975 points

    Hello Murat Celebi,

    Yes, something like the circuit shown in your previous post (although using the ISO124 instead of the ISO120BG) would work.

    Instead of using the 1N5221B I would use the 1N5233B and I hope that Vin will be +/- 1 V maximum (instead of 8 V).

    Hope this helps.

    Best regards,

    Jose

  • 0 in reply to Jose Duenas
    Prodigy 30 points
    I can not ISO120 or ISO124 simulate in Orcad. Vout always has DC voltage, not thing change.
    Please help.
  • 0 in reply to Lance Lan
    TI__Guru* 93105 points

    Lance,

    There isn't a simulation model for the ISO124, but there is one for the ISO120. It is an older model that wasn't developed for Orcad, but instead for MicroSim PSpice. However, Orcad is based on the original MicroSim PSpice and the model should be compatible.

    The ISO120 PSpice model pin functions and external connections may not be obvious to many users because all that is provided is a .MOD file. I don't use Orcad, but I do use a simulator that complies with the Orcad syntax called TopSPICE. I set the ISO120 model up in TopSPICE and successfully ran the model. You can see the results from the simulation below. That verifies that the model is PSpice compatible and should run with Orcad providing the model is connected correctly.

    I added a pin connection table to the schematic to make it easier connect the model. Please note that if you continue to have difficulties applying the ISO120 model that it would be best to contact the Webench > Tools, E2E forum. Officially, TI's Webench Tools group provides support for simulation models.

    Regards, Thomas

    PA - Linear Applications Engineering