May I know which of the instrumental op-amp is suitable to amplify the k- type thermocouple (range: 0-65 degrees celsius) signal?

KeithWalker

Joined Jul 10, 2017
1,919
What you use will depend on the application. For general measurement and control, a LM358 will do the job well but if you need very accurate and repeatable results you will need to use a zero drift op-amp like Texas Instrument's OPA187.
 

Ian0

Joined Aug 7, 2020
3,245
For 0-65°C a thermistor would do the job. If you use a thermocouple in that range, you will have to do a cold-junction compensation, and that usually uses a thermistor.
 

Papabravo

Joined Feb 24, 2006
16,810
If you want to experiment with the instrumentation amplifier concept you can build one from garden variety opamps. It might not have the performance of an integrated device but it will sure beat using a single part. I've included an example. There is a mistake in the text where I transposed the last two digits of the gain resistor. Using the correct value in place of "852" gives the correct answer of 125.95.

EDIT: The diode connected transistors, in the schematic below, will also measure temperature, just FYI.
 

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Ian0

Joined Aug 7, 2020
3,245
The upshot is, if you think you are measuring a temperature around room temperature with a thermocouple, then you aren’t, you are measuring it with whatever device does the cold-junction compensation! And that is either a thermistor or an LM35 circuit buried in your thermocouple amplifier IC, so you might just as well measure it with a thermistor, or an LM35.
To be rather pedantic, if you absolutely must measure 0°C then the LM35 needs a negative supply, so we’re back to the thermistor.
 

LesJones

Joined Jan 8, 2017
3,497
A number of the Analog devices amplifiers have built in cold junction compensation. Is there a reason why you want to use a thermocouple rather than a thermistor or one of the many semiconductor temperature sensors ? Some of the available sensors give the output in digital form so you would not need to use an ADC. What is the whole projects function ? Knowing that would help us to give better advice.

Les.
 

KeithWalker

Joined Jul 10, 2017
1,919
what is cold-junction compensation ??
A thermocouple is based on the Seeback Effect. Because the voltage generated by the thermocouples proportional to the difference in temperatures of its two opposite ends, you need to know the temperature of one end (the cold junction) before you can calculate the temperature at the other end. Usually a thermistor is used to automatically compensate for any change in the cold junction temperature.
 

Ian0

Joined Aug 7, 2020
3,245
Good old Burr Brown OP-07 - and 60p each these days. They were never that cheap!
Unfortunately, there nothing remotely rail-to-rail about them!
 
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