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AD595 Wire Shielding?


I have been successfully using the AD595 to measure the temperature of a thermocouple I have and have confirmed that I can measure the temperature correctly using a myRIO  (DAQ card).  It measures about 0.300 volts when I am at room temperature which is about 30 degrees Celsius.  However, when I turn on other components in my system the analog signal from the AD595 spikes rapidly and begins to oscillate between 1 and 4 volts.

I have wires running directly next to my thermocouple wires that have 24V DC and 5V DC running through them a low amps.  The length of interface between these wires is about 2 feet.  I was wondering how important having shielded thermocouple wires is and if this could be the problem I am experiencing.  I have ran a few tests using different input pins on my DAQ card and confirmed that the problem is not within the DAQ itself.  Could you please point me towards some possible issues that could be causing the thermocouple reading to oscillate like this?



  • Hi Paul,

    Sorry for the delayed response. Usually, when I see something like this, it's an EMI issue. A thermocouple can make a notoriously good antenna. These issues can generally be solved with a low-pass filter on the inputs. Depending on  how your thermocouple is grounded, the filter can be a simple RC, or a differential and common-mode filter (as shown below).

    Resistors should be kept to about 100Ω if possible to preserve accuracy. So generally the trade-off is how large a capacitor you're willing to use.

    Shielding can also do a good job reducing EM pickup, and is worth trying if it's easier for you or if the filter is not enough.

    The other possibility is a ground loop. The thermocouple should only be grounded at one point. For example, if the thermocouple has a grounded tip, but you also connected one lead to ground at your PCB, it would form a ground loop. This can also be an issue with non-isolated thermocouples if they are in contact with a material that has a ground connection. You might try inserting a 1Meg resistor between your thermocouple and ground to see if it helps. Together, that and the filter should look like this picture (from the AD8495 datasheet, but applies to AD595 also).

    Have you considered the next-generation thermocouple amplifier, AD8495? It has better precision, a smaller package, and lower price.

    Best regards,