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This is a 2 different cold junction compensated thermocouple smplifier circuit.

I just don't understand why it needs 1.5MOhm (OP291 Figure 70.), R4 6.04k (OPA335 FIGURE 3.) and how to calculate the resistor value.

enter image description here enter image description here

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  • \$\begingroup\$ humm..asked 1 month ago viewed 88 times \$\endgroup\$
    – tardis
    Dec 17 '17 at 14:18
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These resistors are part of the cold junction compensation, which you need to get an accurate absolute reading. Otherwise the temperature of the cold junction would either need to be temperature stabilized (which was done in the past, using ice baths) or you would get reading that depends on the temperature of your cold junction as well as on the hot junction.

The need for the resistors arise from the temperature coefficient of the diode and the thermocouple being non-linear. These resistors linearise the curve.

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  • \$\begingroup\$ It is not quite correct. Cold junction compensation is accomplished summing temperature dependant diode drop scaled down to TC scale i.e. approx 40uV/C (R2/R3 voltage divider in first curcuit and 24k9/475R in the second one). Try to improve your analysis... what are those two other resistors for? It's a quite common feature too...Also linearity is not an issue given relatively narrow cold junction temperature range \$\endgroup\$
    – carloc
    Dec 26 '17 at 13:18
  • \$\begingroup\$ Thank you for your answer. but, That was not really my question. I don't know why R4(6.04k) and 1.5M Resistor are necessary (see blue circle) Can you tell me why you need it? Thank you very much in advance for your reply. \$\endgroup\$
    – tardis
    Jan 13 '18 at 2:06

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