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Reasons to Ground Thermocouples
Why do I need to ground my thermocouples when I am already measuring them in differential mode?
There are two issues involved here:
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- Static Common Mode Voltage on the Thermocouple Leads:
Your thermocouple leads can develop a substantial potential difference through charge build-up, which may approach or exceed the maximum common mode voltage rejection of your op amp. This can come from the surrounding environment or from the amplifier/multiplexer circuit itself. In either case, grounding the negative thermocouple lead allows this static charge to discharge to ground and thus preserve the common mode voltage at zero. To ground the signal, place a resistor between the thermocouple lead and the analog ground of the card you are using. An ideal range for these resistors is between 10 kOhms and 100 kOhms. Please, refer the tutorial: Field Wiring and Noise Considerations for Analog Signals.
- Injected Current from the Amplifier/Multiplexer Circuit:
Every op amp generates a Reverse Bias Leakage Current, which causes charge to build up on its positive and negative input leads. This is typically negligible, but it can build up over time, leading to effect listed above. Also, as the multiplexer switches between pairs of input leads, a short-time current (charge injection) is introduced into the new pair of leads. Since thermocouple voltages are typically about 1 mV, these short-time fluctuations can measurably affect the input signal. Again, the solution is to connect the negative thermocouple lead to ground, allowing the injected charge to discharge to ground and not appear at the positive thermocouple lead.