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[SOLVED] RTD and/or TC input circuit

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Thoma HAUC

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Hello,

I start to design a RTD measurement circuit based on a current-loop signal conditioning. And now, I am wondering if it would be possible, with some modifications, to measure also TC.

This functionality should work without the use of relay. I know it is possible as some companies offer this king of input.

Thank you in advance

Thoma
 

hamed8419215

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The resistance of an RTD changes with respect to temperature but the output of TC is EMF. i think that you should design 2 difference circuit.
 

Thoma HAUC

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hamed8419215,

I agree with you, RTD has a temperature depending resistance.
So, imagine, you have a known current that flows through the RTD, then you have an EMF that appears at its terminals.

Finally, in both cases (RTD & TC), you measure an EMF to estimate a temperature.

Thoma
 

hamed8419215

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but the result output voltage in an RTD measurement circuit is more larger than the output EMF of an TC. for example if you use a current source of 500ua with a PT100, you have about 192.5 uv per centigrade. but the maximum output of a TC (for example in K) is not larger than 60uv per centigrade.
ALSO in TC you should compensate reference temperature. and you know that an RTD is approximately linear over it's temperature range but TC not.
 

FvM

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I know it is possible as some companies offer this kind of input
True, what are you exactly asking for? State-of-the art temperature transmitters are usually processor based modules, with variable range and digital linearization and calibration. You won't be able to implement their features in a simple analog-only design. In my opinion, the days of analog linearization are over, because it's so much more complicated than the digital way. For RTD, there's a simple second order linearization circuit based on positive feedback, for TC amplifiers, I'm not aware of a similar solution.
 

Thoma HAUC

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hamed8419215,

You are right; but currently the RTD input is based on:
- a 200µA current source (around 80µV/°C for a Pt100),
- a ADC with integrated PGA and
- a microcontroller to control the ADC and to linearize the RTD response.

I know also that TC has ratio as low as 10µV/°C (type B). But as I have a PGA in the ADC, it would not be a problem.

FvM,

I am asking for advices for the analog part of the design. As you can read above, there will be a microcontroller for linearization.
So, my idea was to foresee all required hardware on the analog side to be able to measure cold-junction temperature and also provide the linearization.

Thanks, both, for your interest in my topic.

Thoma
 

hamed8419215

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all right. if you have such thing, then there is no problem. in other word if you have a current source, a PGA, an MCU, an integrated temperature sensor and an A/D, you can measure temperature with both RTDs and TCs.
 

Thoma HAUC

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Hi all,

Regarding the wiring, what will be the best configuration to be able with the same circuit to measure RTD and TC?

Thoma
 

hamed8419215

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if you want to work completely microprocessor based, you need a PGA and A/D with 2 channel.
 

Thoma HAUC

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Hi all,

On the left, you will see the measurement principle for the RTD. RTD value is estimated using the below resistance as reference.

Now if I remove the RTD, I will have the circuit on the right.
Another multiplexer connection is available to measure the cold-junction temperature.



What is the best solution to connect the thermocouple on the existing circuit? Should I foresee some modifications?

Thank you in advance for your hints.

Thoma
 

FvM

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You can e.g connect the TC to the upper differential input. Most likely, you would want to ground one side to assure a defined common mode voltage. Also a small current source/high ohmic resistor as open circuit detector is usually wanted.
 

Thoma HAUC

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Hi FvM,

Do you think that defining a common mode voltage through a 10k resistor is acceptable?
And how large should by the line break detection current?

Thank you in advance.

Thoma
 

FvM

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The question is, if besides amplifier input currents, interfering currents exist that may possibly overdrive the amplifier. 10k sound generally reasonable. About 1 uA should be suffcient for open circuit detection, assuming the input current is clearly lower.
 

Thoma HAUC

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Hi FvM,

Are there any contraindication to use a higher current (200µA)?

Thank you in advance.

Thoma
 

Thoma HAUC

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Hi FvM,

I lost track of wires resistance that can greatly impact the over all accuracy.
I suppose I have all information to design my measurement circuit.

Many thanks, guys.

Thoma
 

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