Hi all,
If you have the signal conditioning circuit that is applicable to all types of thermocouples , then send me its link. I am making DAQ card which has a 10 bit ADC. The signal conditioning should be perfect enough that the output voltage from
the signal conditioning circuit is in the range of 0 - 5 volts that can be applied to ADC. The input of the signal conditioning
circuit for my application is -10mV to + 80mV. I also read tutorials for signal conditioning of thermocouples, that
also helped me but i want signal conditioning circuit with its explanation.
Waiting for your useful reply
Muzammil,
Conditioning a thermocouple over an input range of -10mV to +80V requires an instrumentation amplifier and can be accomplished in a couple of different ways. The other thing that will be needed is a temperature sensor for cold-junction compensation. From your question it looks like you are mainly concerned with the signal conditioning portion of this circuit and not the CJC; therefore, my response below will address only the signal conditioning portion.
Most instrumentation amplifiers have a common mode input limit from the rail and require some DC headroom to operate in a linear fashion. As an example, you could use the INA333 with the same +5V supply as the ADC; however, you need to center the common mode inputs about your +5V to be able to maximize the gain. Likewise, you must pedestal the output at the Vref pin to ensure that the -10mV signal does not saturate the output into the ground rail. Most thermocouple conditioning circuits need to be able to detect an open thermocouple. One way to accomplish this is to use a divider to center the common mode and then pull the positive rail up to supply through a large resistor. With a thermcouple present the pull-up resistor will have little influence; however, in an open condition the difference between the inputs will be large enough to drive the output into the supply rail. Here is the signal conditioning circuitry for the INA333:
If you do not want to have to bias the common mode inputs and would like to swing close to ground, consider an INA326 implementation:
You can bias the reference in the same way as was done with the INA333 circuit; likewise, the pull up resistor on the + input is for an open thermocouple condition.
I hope this helps you get started.
Matt
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