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The voltage offset variable resistor is the 10 kΩ rheostat connected from pin 3 of the "top" LM324 to GND.

It is unlikely that you actually have a rheostat, you probably have a potentiometer with 3 connections. You should link the wiper (usually the centre one of the 3) to one of the ends and then connect as per the diagram. (That is so that if the wiper connection lifts off due to a spec of dust or similar, you get 10 kΩ rather than infinity.)

Hello Robert,

Thank you for your explanation.

Now I see (at least so I expect) that I completely missed a couple of things... :-)

In fact I have a 10K potentiometer (actually a 10K trimpot), but I (wrongly) thought that it's functionality was to simulate the resistance at temperature extremes - following the Trystan example -10ºC and 115ºC.

With this in mind I was not understanding how this variable resistance would be in place after the initial calibration.

Now, with your clarification, I understand that this variable resistor must be always in the circuit, even after making the initial adjustments. In what position should it be? The last one with the higher point mark, this first one (the minimum) or any other?

To simulate the PT100 minimum and maximum resistance values I should use a second variable resistor and the product datasheet to get those values, right?

Thanks for your clarification.

The way the circuit works is the voltage appearing across your platinum resistor, after it's been selected by the multiplexer, is amplified and fed into the input ADC. But before then, the first step is a unity gain buffer stage. Unfortunately the "reference" temperature (at which you expect a count of zero out of the ADC) is not 0 V, but offset. The second amplifier therefore has two inputs, and amplifies the difference between the two inputs. So its second input, is actually the voltage that your PT100 probe gives you at your "zero count" temperature. There are calibration instructions in the article.

With your help, I have my prototype reviewed and now operational.

I already did the calibration for both minimum and maximum temperature.

Tomorrow I will test it against ice and boiling water to see how it works.

Thanks again for your help.

Hi saturano,

​i am very interested in your prototype fpr 4x pt100.

can you explain your changes to the "RTD Temperature Sensing Module" (here) above...

Thanks, Marc

This could be interesting to, but very expensive...

http://sensors.egnite.de/querx-pt/