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Two Thermoucouple circuits interfering with each other

Other Parts Discussed in Thread: INA126, INA2126

Hi I have a circuit board which has inputs for two thermoucouple K_type clamps. They are fed into two seperate INA126 amplifiers that run off +5V. The front end is biased to 2.5 volts to keep the circuit in the common mode range. Both circuits work fine when used seperately, however when both clamps are connected to a pipe for example the circuits interfere with each other and I get erronious results. Does anyone have a solution to this problem? I imagine its a common problem when two thermocople inputs are required.

Thanks

  • Hello Jerry,

    You are most likely shorting the thermocouples together since they're both connected to a conductor (a pipe).  Here is a link to an article that may be of help:

    http://www.omega.com/techref/das/tcshort.html

    The last paragraph discusses the use of non-conductive paint, tape, or epoxy to electrically isolate the sensors.

    I hope this helps!

  • Pete,

    thanks for the reply. Yes, I know that the problem is due to the connection between the thermocouple junction and the pipe.The problem is that it is a clamp type thermocouple.  I cant find any isolated types. Putting tape or epoxy on the clamp is not feasible as this is a production design and paint or tape will wear off in the field.  The low side of both channels are connected together. I thought that would alleviate the problem, but it doesn't.

  • By the way- I did solve this problem. The problem was that the two op amps (INA2126) used to interface the thermocouple  had their input biases tied together.  Since the leak off current required is extremely low I just added a 1M resistor in series with the bias connection and now both circuits read correctly.
    Thanks for the help