I've been working with signal conditioning, thermocouple out specifically. I'm working with a span of 450 °C, that is, from -50 °C to 400 °C. For those values of temperatures I get, according to the table, -2.431 mV and 21.848 mV. The problem is that I need to convert that into 0-5V signal. I am thinking of using the AD620, which is an Instrumentation Amplifier. It's been impossible for me to convert -2.431mV into 0V. Do you have any suggestions about this?
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4\$\begingroup\$ There are special-purpose built devices for thermocouple signal conditioning, so you should do some more research on components. It may help to consider devices that can do the thermocouple linearization for you. \$\endgroup\$– user2943160Commented Jun 27, 2016 at 2:19
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\$\begingroup\$ Strongly agree - unless cost or space are prohibitive, an off-the-shelf thermocouple signal conditioning module or 'transmitter' will get the job done much better than rolling your own. \$\endgroup\$– nekomaticCommented Jun 27, 2016 at 11:16
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\$\begingroup\$ How much accuracy do you need? This determines which of many techniques might work for you. For example, we do a very simple thermocouple transmitter that converts K-type TC over the range of 0C - 800C to a variable current that ranges from 1-5 mA (4-20 mA /4). It's accurate to about 1%, which is suitable for our needs. But it is NOT lab-grade accuracy. \$\endgroup\$– Dwayne ReidCommented Jun 29, 2016 at 5:17
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\$\begingroup\$ I do not need much accuracy. As long as I get an acceptable error it is okay. I have gotten the ICs AD620 and LT1013, do you think they might work for this purpose? I'm having problems with that negative voltage (-2.431mV). I need to be able to design the circuit using only positive voltages. \$\endgroup\$– user115094Commented Jun 29, 2016 at 5:22
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2 Answers
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Unless you have the "cold-junction" offset handled by some other method which you did not mention in your question, it is not recommended to use a simple op-amp to "condition" a thermocouple signal.
Analog Devices makes a range of specialty "Thermocouple Interface Amplifiers". Highly recommended to use one of those. There are some "break-out boards" which feature the complete thermocouple conditioning ready to use.
Ref: https://en.wikipedia.org/wiki/Thermocouple#Requirement_for_a_reference_junction
answered Jun 27, 2016 at 2:52
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\$\begingroup\$ I have handled the cold-junction by using a LM35. I am pretty limited regarding electronic components such as thermocouple interface amplifiers, so I have to get an approach by using other components easier to get. Since I have AD620, I am trying to handle the situation because I think it is better than using LM358 and LM741 which are not for this application. Anyways, I will upload a schematic about my circuit. \$\endgroup\$– user115094Commented Jun 27, 2016 at 3:30
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The building block pieces you'll typically need are:
- Cold junction (a.k.a., reference junction) compensation
- Voltage amplification circuit (typically an instrumentation amplifier)
- Voltage linearization circuit (optional)
- Band limiting filter (low pass filter) to mitigate the the corrupting effects of high frequency signal aliases in the digitized data (recommended)
- Analog-to-digital (ADC) conversion
I agree with @user2943160 that a commercial off-the-shelf (COTS) thermocouple amplifier IC is the simplest way to go, rather than trying to build the cold junction compensation, amplifier circuitry, etc. from scratch.
(n.b. COTS thermocouple amplifier ICs typically do not provide the band limiting filter function/circuitry. The filters are external to the IC and are designed/provided by the circuit designer.)
For what it's worth, Analog Devices published a series of eight short videos on YouTube titled Thermocouple 101 that are worth watching in my opinion. (HINT: This video series describes how to set the 2.5 VDC common mode input voltage at the instrumentation amplifier's inverting and non-inverting inputs, to manage the -2.431 mV voltage.)
