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ISO7741: Separating GND using the Isolation solution

David__
Genius 3585 points
Part Number: ISO7741
Other Parts Discussed in Thread: DAC60504, OPA551, , ISO224

Hi, I'm using TI's DAC60504 to create a bipolar signal (ref. slaa869) and a Howland current pump.

This bipolar signal is fed to the Op Amp's input

Currently, both the OPA551 Op Amp (+30V, -30V supplied) and the DAC DAC60504 is tied to the same GND;

GND - The load resistor connected to the Op Amp - DAC GND

However, I need to use separate the GND and I noticed the Isolation solutions.

Since this current pump drives +30, -30V with 60mA max, can I use the Isolation solutions to use separate GNDs?

First, I thought to add a unity gain buffered GND to the load, but I thought the Isolation solution could be better.

  • 0
    TI__Guru 54155 points
    Hi David,

    Could you please confirm which two GNDs or devices you would like to be isolated?
    ISO7741 is a digital isolator and it can be used to isolate two devices with digital interface or I/Os. In your application, ISO7741 can be used to isolate the SPI interface between MCU and DAC. Please refer to section 9.2 in ISO7741 datasheet to see an example use case for ISO7741.

    Let me know your inputs so that we can suggest you the suitable device accordingly, thanks.


    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    Genius 3585 points

    Hi, Koteshwar.

    Koteshwar Rao said:
    ISO7741 can be used to isolate the SPI interface between MCU and DAC.

    I think you are mentioning this figure, right?

    However, that is not what I want to do. ISO7741 has two GNDs; GND1 and GND2.

    For my microcontroller's GND and DAC's GND, I want that to be connected on GND1.

    Whereas, if you take a look at my Howland Current Pump circuit,

    there is a GND symbol which is connected to the load. I want that to be connected as GND2.

    This is how I want to separate the GNDs. Is this applicable? Or do you suggest other methods?

  • 0 in reply to David__
    TI__Guru 54155 points
    Hi David,

    I am not too familiar with the Howland Current Pump circuit but I think I understand your requirement better now. Am I correct to say that you would like to isolate the DAC from Op-Amp circuit? This means that DAC will be on one GND while Op-Amp circuit on another GND.

    The interface between DAC and Op-Amp circuit is analog and ISO7741 is a digital isolator and can't be used to isolate analog signals. We do have analog signal isolators as well like the ISO224. To better comment on the suitability of ISO224 for your application or to suggest a suitable device, I will notify the analog isolators team to look into this E2E post.

    Meanwhile, could you please let me know if there is any particular reason for not isolating between MCU & DAC instead of DAC & Op-Amp? It is a common practice to isolate between MCU and data converters when isolation is a requirement but do note that there are applications where isolating an analog signal is necessary. That's why I wanted to understand the reason in your case. Thanks.


    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    Genius 3585 points

    Thanks, Koteshwar.

    Koteshwar Rao said:
    could you please let me know if there is any particular reason for not isolating between MCU & DAC instead of DAC & Op-Amp? It is a common practice to isolate between MCU and data converters when isolation is a requirement but do note that there are applications where isolating an analog signal is necessary.

    Well, I don't have a specific reason for not isolating the DAC and the MCU. I tested the TI's DAC with multiple launchpads like the MSP430, MSP432 launchpad.

    When the DAC and the launchpad used the same GND, it worked nicely for me. So I didn't really felt the necessity. 

    Since you mentioned this is a common practice to isolate the GND between the MCU and the DAC, may I ask the reason why for doing that?

    After your reply, I'll close this thread. Thanks for suggesting an alternative, too.

  • 0 in reply to David__
    TI__Guru 54155 points
    Hi David,

    The primary reason for isolating in the digital domain is that it is relatively easy to isolate a digital signal and reproduce it accurately without any issues than an analog signal directly. Isolating an analog channel directly will pose many challenges include the need to maintain linearity of analog signal to reproduce it accurately. In digital, the absolute voltage reproduced is not of any importance as long as the states are reproduced accurately. Due to less complexity involved, digital isolation will be lower in cost to implement compared to analog isolation.

    Please refer to the below list of reference design that implement isolated analog outputs. Thanks.

    www.ti.com/.../TIDA-00231
    http://www.ti.com/tool/TIPD155
    www.ti.com/.../TIDA-00118
    http://www.ti.com/tool/TIPD119
    www.ti.com/.../TIDA-00760


    Regards,
    Koteshwar Rao