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ADS114S08: ADS114s08 Vbias with a grounded therocouple

Mahmoud Zannerni62
Intellectual 655 points
Part Number: ADS114S08
Other Parts Discussed in Thread: ADS1220, ADS124S08, LM27762, ADS1255

Hello,

 

I am using ADS114s08 for a thermocouple measurement, and would like to ask if the Vbias voltage should work as expected when using an Earthed thermocouple.

 

As far as I know there are three different scenarios with thermocouples, I will talk about measuring the temp of a metal tank as an example.

1. Ungrounded/insulated TC tip, this should be fine.

2. Grounded TC tip but the tank is isolated from earth, this should be fine too.

3. Grounded TC tip and the tank is connected to earth! I am not sure what could happen in this case! And if the Vbias is not expected to work what is the solution for this case?

 

I am using a unipolar supply, and trying to avoid going to the bipolar route.

Any advice will be appreciated.

 

Regards,

Mahmoud

 

  • 0
    TI__Mastermind 44720 points

    Hi Mahmoud,

    Grounded thermocouples will introduce ground loops into your system and it will make very difficult to use the ADC's Vbias voltage, since Vbias MUST be at the same potential as the thermocouple. This means you'd have to use a bipolar supply and make sure the supply grounds (i.e. the ground potential to which the +/- 2.5V supplies are referenced) are also referenced to the same ground potential as the thermocouple, which may not be practical when there is a long length of wire between the thermocouple and your PCB. When designing your system in this way, you MUST also ensure that you isolate the thermocouple and ADC circuitry from any other systems that has the potential to operate at a different ground potential (earth ground is not a low impedance ground, so two systems with different earth grounds may be operating at different ground potentials). Therefore, unless your system is entirely self-contained and battery operated, you'll need to add isolation (most likely digital isolation) between your local ADC and any other equipment that communicates with your circuit.

    In my opinion, it would be easier to use a thermocouple that is thermally conductive, but electrically isolated from the outside world, so that you avoid the problems with ground loops, and perhaps avoid having to add isolation between the ADC and MCU (for example).

    A note of caution... In use case #2, it is probably not safe to assume that the object that you are connecting the thermocouple to is isolated from earth ground. A person who comes in contact with the tank could electrically ground the tank and get shocked, so it would be better to build the safety measures into your system. If you don't have control over what kinds of sensors are connected to your system, or if there is a possibility of human error to connect a grounded sensor, then you may have to add isolation into your systems anyways to protect end-users.

  • 0 in reply to Christopher Hall
    Intellectual 655 points

    Hi Chris,

     

    Thanks for your detailed answer.

    Sorry, I didn't mention that the ADC is isolated, I am using an isolated DC-DC to power the analog circuit.

    I tried to use a grounded thermocouple with Vbias in my lab and it worked as the floating one, should this work in factory case if there are no ground loops (The system is connected to earth by one point only)?

    I am thinking also of connecting the earth to the ground of the ADC and keep using the Unipolar supply? would you expect this to be okay?

     

    Regards,

    Mahmoud

  • 0 in reply to Mahmoud Zannerni62
    TI__Mastermind 44720 points

    Hi Mahmoud,

    If your thermocouple connection header is your single-point earth ground connection, then perhaps this would avoid the issue with earth ground loops. However in this case, if you do not connect a grounded sensor, then your ADC circuitry would not have an earth ground connection at all - it would just be floating. Also, you wouldn't be able to connect multiple grounded sensors to the ADS124S08, so it would be more cost effective to use a lower channel count ADC, such as the ADS1220, in this way.

    I still think you would need to use bipolar supplies with the ADS124S08 (or ADS1220) though... However, you wouldn't need to enable Vbias, as long as the +/- 2.5V supplies use the thermocouple's earth ground as their ground reference. See Figure 17 in http://www.ti.com/litv/pdf/sbaa274.

    Bipolar supplies could be fairly easily generated off of a 3.3V or 5V supply using the LM27762.

    Alternatively, we do have an ADC with a special front-end implementation that would allow you to use the internal PGA and ground one of the input pins: the ADS1255. This ADC could be operated off of a single supply and connected to a grounded thermocouple, but this ADC does not have as high of an input impedance as the ADS124S08.

  • 0 in reply to Christopher Hall
    Intellectual 655 points

    Hi Chris,

    Thanks for suggesting the LM27762, will consider this option.

    I have a question about

    Christopher Hall said:
    If your thermocouple connection header is your single-point earth ground connection, then perhaps this would avoid the issue with earth ground loops. However in this case, if you do not connect a grounded sensor, then your ADC circuitry would not have an earth ground connection at all - it would just be floating

    Will keeping the ADC circuitry floating cause any issues? if I am using a floating thermocouple? I have a connection between ADC gnd and earth through a high voltage cap.

    Regards,

    Mahmoud

  • 0 in reply to Mahmoud Zannerni62
    TI__Mastermind 44720 points

    Hi Mahmoud,

    The circuit's ground potential can float with respect to earth ground, but all of your ADC related circuitry much be referenced to that "floating ground" in order for the circuit to function.

    Having a floating ground potential can introduce other problems though...

    • As your ground potential floats, you are essentially injecting a common-mode signal into all of your circuitry, which may lead to common-mode noise issues.

    • Also, protecting circuitry with a floating ground can be a challenge as you do not have an earth ground that can shunt energy away from sensitive components. A capacitor to ground will help with shunting some of the transient energy, but this is not as good as a direct connection. 

    These may or may not be reasons to avoid having a floating ground, but they are things to be aware of and to try to mitigate.

  • 0 in reply to Christopher Hall
    Intellectual 655 points

    Hi Chris,

    Thanks for the information.

    I would like to ask about having more than one earthed thermocouple.

    You menthioned earlier "Also, you wouldn't be able to connect multiple grounded sensors to the ADS124S08"

    I don't see why, the inputs of the ADS124s08 are differential and the measurment is taken for the channels one by one.

    So only one earth connection will be connected to the ADC at the same time.

    Regards,

    Mahmoud

  • 0 in reply to Mahmoud Zannerni62
    TI__Mastermind 44720 points

    Hi Mahmoud,

    If you introduce a second earth ground connection to your system then you run the risk that these earth grounds could be at different potentials. For small potential differences, you'll have a ground loop in your system. However, if the voltage potential is larger, then its quite possible to damage sensitive circuitry.

    In order to use multiple grounded thermocouples, I would recommend implementing channel-to-channel isolation so that each thermocouple's earth ground is isolated.