I need a quick sanity check... I once read somewhere that since thermocouples have a low output impedance, they're not very susceptible to noise problems. I understand why thermocouples have low impedance, they're just two conductors bonded together. But, once you connect the thermocouple to an amp with a high input impedance, doesn't this pretty much eliminate the benefit of the thermocouple's (or any source) low output impedance? Am I missing something fundamental here?

 

Actually the low impedance of the thermocouple dominates the input impedance when connected to a high impedance input to an amplifying stage.

hgmjr

 

The lower the impedance the higher the influence. I am guessing that it has to do with having more power available at a given time than a higher impedance component. This is evident when dealing with audio equipment.

 

But, once you connect the thermocouple to an amp with a high input impedance, doesn't this pretty much eliminate the benefit of the thermocouple's (or any source) low output impedance?

The high impedance (here really a resistance, as it's pretty much all DC) preserves the signal out of the thermocouple by loading it so lightly that the output voltage is not affected.

Take the same junction and apply the output to our favorite op amp, the LM741. Set it up as a buffer, and see what the output voltage from the op amp is.

Now substitute a good modern op amp like the OPA134. See what it has for an output.

That will illustrate the difference between a 200K input and one that is 10^13.

 

After reading my orginal post again, maybe a better way to phrase my question would be: Doesn't the high input impedance of a modern amplifier increase the EMI noise susceptibility in the TC circuit, regardless of the fact that the TC has a low output impedance?

 

Think of the situation as two resistances in parallel - the effective resistance is always less than the lower of the two in parallel.

 

If the thermocouple has one ohm of resistance and the op-amp has a million ohms, one ohm in parallel with a million ohms is still less than one ohm. They are, in fact, in parallel.

 

The high impedance (here really a resistance, as it's pretty much all DC) preserves the signal out of the thermocouple by loading it so lightly that the output voltage is not affected.

This is more of what I was getting at. If the TC is loaded lightly, wouldn't an undesirable noise source (inductively coupled into the lines) be loaded lightly as well - showing up as error in the signal?

 

The thermocouple IS a load, and it is NOT a light load.

 

I would think the low TC resistance just ensures the common mode signals at the TC junction are effectively the same. The real problem is more likely to be the induced noise created in the conductors carrying the junction emf to the amplifier input. Presumably that's why, apart from using true differential input amplifiers one does things like twisting the signal conductors together and shielding the signal wiring to minimise the induced noise voltage from external electric and magnetic fields.

 

I guess I need to say I was thinking of the thermocouples I use. They don't have 50 foot wires and differential instrumentation amplifiers...just a slow time constant so the mains hum doesn't bother the readings.

 

I would think the low TC resistance just ensures the common mode signals at the TC junction are effectively the same. The real problem is more likely to be the induced noise created in the conductors carrying the junction emf to the amplifier input.

I found the accompanying booklet to a two day course I attended on grounding and shielding. Paraphrasing from the booklet, a low impedance source driving a high impedance load will have minimal susceptibility to capacitively coupled noise, but will have risk of error from inductively coupled noise sources. Basically what t_n_k said. Thanks everyone for the replies.

 

That is true. It will work as an antenna. Proper termination, and/or shielding can reduce the noise involved, however.

 

One can also come unstuck with respect to noise when good grounding practice is neglected. 'Unplanned' ground loops are often a source of unexplained noise &/or "hum" - and they [ground loops] aren't always obvious.