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Topic: how to amplify a thermocouple signal? (Read 2 times) previous topic - next topic

Grumpy_Mike

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I realize I might lose some accuracy by going this route, which I think I can mitigate by averaging multiple readings

No .... taking an average only compensates for random noise. What you will have is systematic inaccuracies. These are caused by the DC offset on the op amps.

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I *do* see on the schematic where the max chip uses two op amps

Yes but do you see all those switches on the inputs. They are used to compensate for the DC offset on the op amp's input.  If you try to get a lot of gain from an amplifier at DC then you amplify the DC offset as well. This causes the amplifier to saturate without even having any signal from the thermocouple. Any circuit used must compensate for this.

Why don't you try an google a schematic?

BetterSense

I don't mean to derail the conversation, and I understand doing things for teh lulz, but I hate thermocouples and would suggest using RTDs if possible.

Jack Christensen


I *do* see on the schematic where the max chip uses two op amps, but I have no idea why.


The second op amp has its gain set to one, so it would appear to be a buffer.  I'd plan on some heavy studying up on op amp theory and application...
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

retrolefty


I don't mean to derail the conversation, and I understand doing things for teh lulz, but I hate thermocouples and would suggest using RTDs if possible.


While I don't hate TCs, having worked with them for decades in an oil refinery, I tend to agree that TCs are not the best choice for most applications. As well as needing cold junction compensation they also suffer from a small amount of non-linearity over there total measurement range, so absolute accuracy is not all the great unless you add curve fitting functions. Their main claim to fame is that they are fairly inexpensive to manufacture and have very high maximum temperature capability. However if you don't need the very high temperature feature then RTDs and some of the newer semiconductor temp sensors can be much more accurate and a lot easier to interface with.

Lefty

Graynomad

How high a temp can RTDs go to?

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

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