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I have circuit schematic with LTC6081 for thermocouple, but LTC6081 isn't available in my country. Can I use LM324 instead of LTC6081?

enter image description here

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  • \$\begingroup\$ If your user name is your real name, I find the opening statement ("LTC6081 isn't available in my country") hard to believe as-is. You probably mean to say it's too expensive or something like that. \$\endgroup\$ Commented Oct 8, 2015 at 15:33
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    \$\begingroup\$ What temperature range are you measuring, and what accuracy and resolution (they're not the same) do you need? For some low-performance applications you can use an LM324 (with an offset necessary), but for those applications you probably wouldn't need a thermocouple. So fill us in. \$\endgroup\$ Commented Oct 8, 2015 at 17:45

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Maybe, but it will probably not perform as good. As the datasheet of the LT1025 tells us in "Amplifier considerations":

Thermocouple amplifiers need very low offset voltage and drift, and fairly low bias current if an input filter is used.

The LM324 has typically 2mV offset voltage (instead of 70µV of the LTC6081), and a typical temperature drift of 7µV/K (compared to 0.8µV/K maximum on the LTC6081). Also the bias current with 45nA is on the higher side (pA on the LTC6081 and <1nA on others).

But it also depends on your application, will the temperature of the amplifier stay mostly constant? Then temperature drift of the offset voltage isn't that much of a problem.

(Are you using S or R type thermocouples? Then you will be in a world of hurt with the LM324. As pointed out, the schematic assumes a K type thermocouple, thanks @RespawnedFluff)

You can get around the offset if you do some offset correction.

If you can get the LT1025, then maybe some of the recommended amplifiers are available.

Chopper stabilized amplifiers work quite well for thermocouples. Something like the TLC2652A or OPA2735 if TI parts are available.

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  • \$\begingroup\$ Regarding thermcouple type, the very same LTC6081 datasheet says (in all caps, as you know): "SENSOR: OMEGA 5TC-TT-K-30-36 K-TYPE THERMOCOUPLE". At least that's easily substituted with something cheap (list price for that specific model might be a shocker for some; it's $46). \$\endgroup\$ Commented Oct 8, 2015 at 15:55
  • \$\begingroup\$ @RespawnedFluff well yeah, that's true. And it's connected to the K of the LT1025... Guess I wasn't quite perceptive enough. \$\endgroup\$
    – Arsenal
    Commented Oct 8, 2015 at 15:59
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The LTC6081 has a very low offset voltage. Which you want for a thermocouple. (The LM324 not so good...) Can you get an OPA277? Though it has a bit more bias current... which could be an issue with the 1Meg series resistors. (And why are those resistors in there?)

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  • \$\begingroup\$ Re "(And why are those resistors in there?)" I guess you should ask LT. The schematic uses the signature colors from their datasheets/appnotes, so I'm like 99.9999% certain it's lifted from one of those. \$\endgroup\$ Commented Oct 8, 2015 at 15:34
  • \$\begingroup\$ Yeah I guess they are getting some LP filtering from the 1 Meg and 1 uF. The opa277 has 1nA of bias current.. which is 1mV with 1 Meg ohm. (one could add more filtering in the feedback loop.) \$\endgroup\$ Commented Oct 8, 2015 at 15:41
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    \$\begingroup\$ Actually it's said in the datasheet (in all caps) "1M RESISTORS PROTECT CIRCUIT TO ±350V WITH NO PHASE REVERSAL OF AMPLIFIER OUTPUT". Mystery solved. \$\endgroup\$ Commented Oct 8, 2015 at 15:41
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    \$\begingroup\$ @RespawnedFluff gotta love the all caps datasheets :D \$\endgroup\$
    – Arsenal
    Commented Oct 8, 2015 at 15:49
  • \$\begingroup\$ Yeah the more I look at it the less I like those series resistors... more different metal contacts to screw up the cold junction compensation. \$\endgroup\$ Commented Oct 8, 2015 at 15:51

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