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ADS1261: Suggestions regarding selection of Isolator

Sahil Nayak
Prodigy 90 points
Part Number: ADS1261
Other Parts Discussed in Thread: ADS8598S,

Tool/software:

Hi Team,

We are designing a data acquisition system that shall comply with the DO-160 military standard. In our system, we will be reading the data from the following sensors:

  • 32x channel PZT sensor
  • 3x channel RTD sensor
  • 9x channel Strain gauge sensor


From a protection point of view, we are looking to add isolation in our system. While exploring the concept of isolation, we came across two types of isolation: digital isolation and analog isolation. As per our understanding, we will require analog isolation between the sensor and the ADC. Could you please help out select the best isolator that meets our isolation as well as our channel needs?

We are planning to use ADS8598S ADC for the PZT sensor and ADS1261 for the RTD & Strain gauge sensor. Attached is the block diagram of the sensor and ADC interfacing (without isolator).

Please let me know if you require any further information to assist us.

Thanks & Regards,
Sahil Nayak

  • 0
    TI__Guru 52106 points

    Hi Sahil Nayak,

    How did you determine that you need analog isolation on your inputs? This will require one isolator per input, so that is a minimum of 32+3+9 = 44 isolators. But the RTD and strain gauge sensors are usually differential measurements, so that requires 2x isolators per input, or 56 (I am not sure if the PZT output is differential or single-ended. The ADS8598 is single-ended input so I assume the sensor output is single-ended).

    Using digital isolation you would only need 6x isolators, according to your diagram, because there are only 6 ADCs. It might also be possible to use fewer digital isolators if you share the SPI bus and have dedicated CS pins

    Please let us know

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 90 points

    Hi Bryan,

    Thank you for your quick response.

    We are unsure of how the isolators are to be used in the application.

    The PZT output is single-ended, while the RTD & Strain gauge sensor outputs are differential.

    Since I have detailed my application, could you please let us know what isolation should be required (digital or analog) and how that is determined? Let me know if you require any more details.

    Also, if we are going with analog isolation, do we have a multi-channel isolator available?

    Thanks & Regards,
    Sahil Nayak

  • 0 in reply to Sahil Nayak
    Prodigy 90 points

    Hi Bryan,

    In addition to the above query, I wanted to ask another query.
    Our application is a Data acquisition system whose input voltage is 28 VDC.
    Do we really require the isolation for the PZT sensor and RTD & SG sensors? Or is it fine if we don't keep isolators?

  • 0 in reply to Sahil Nayak
    TI__Guru 52106 points

    Hi Sahil Nayak,

    Can you explain what you mean by 

    Sahil Nayak said:
    Our application is a Data acquisition system whose input voltage is 28 VDC.

    Do you mean the supply voltage is 28V? Or are all of your sensors biased with 28V?

    I'd expect the RTDs to be biased by a current source, so their output should be in the range of the ADC by default e.g. <5V

    Not sure how you plan to bias the strain gauges, but a common method is just to use the ADC supply (AVDD) to bias the bridge. So again, this would by default be in the range of the ADC

    Your PZT sensor path looks like it has a resistor divider that I assume takes the output voltage and divides into the input range of the ADS85xx. I then also assume you can tune this divider to ensure the input is always within the range of the ADC

    Therefore, I am not sure I understand why you would need isolation. Strain gauges and RTDs can't have a common-mode difference between them if they are biased by the same ADC on the same ground. Not sure about the PZT sensors, so maybe ch-ch isolation is required? That is something you would need to determine. When you do please let me know and we can make a suggestion

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 90 points

    Hi Bryan,

    28V DC is the input volatge to our entire system.

    We are not sure if the isolation is required or not for the sensors. That's where we are seeking suggestions from TI on whether we should have isolation or not.

    If the isolation is required, which type of isolation will be best for our application?
    Also, please suggest mutli channel isolators that helps to reduce the overall system complexity, if available.

    Thanks & Regards,
    Sahil Nayak

  • +1 in reply to Sahil Nayak
    TI__Guru 52106 points

    Hi Sahil Nayak,

    Again, it's not clear to me what you are trying to isolate, or why? Are you concerned that the 28V module voltage will be applied to the inputs and damage the ADCs? If yes, then I don't think you need an isolator to solve that problem, you can use current-limiting resistors on the low-voltage signals (SG and RTD). It already looks like you have a voltage divider on the PZT channels, so the divider should reduce the magnitude of the overvoltage event somewhat (I don't know how much since I don't know what the divider values are) as well as limit the current into the pins. Also the ADS8598S can already tolerate +/-10V input signals.

    Will there be high common-mode differences between the sensors? Then you might require analog isolation on the inputs. As I mentioned in my previous post, I wouldn't expect a common-mode difference between sensors for the RTD and SG. I don't know as much about PZT sensors, so I will defer to you if that is required.

    Ultimately, I don't think I can tell you if you need isolation in your system, that is something you need to decide based on your system requirements.

    If you do decide you need digital isolation, a good option is https://www.ti.com/product/ISO7741. This is a 3 forward / 1 reverse isolator, but there are options for more or fewer channels, as well as different configurations e.g. 2 forward / 2 reverse. This is a good device to start with, but you can also reach out to the Isolation E2E forum if you have specific questions about isolation

    -Bryan