RTD Temperature sensor module OP problem

Hello everyone,

i’ve built the temperature sensor module with this guide and the linked schematics

I have modified the resistors RA/RB to 1K and 1.5K. That should result in a amplification for my sensor of 1.5 and a constant voltage V1 of 300mV. I power all components over 3.3v.

I have a problem with the upper voltage follower which produces at his output not constantly 300mV. If my output at the difference amplifier end stage gets highter than ~400mV the output voltage of the upper LM324 follower gets also constantly higher. The result of the end stage is mathematically correct to 1.5*(V2-V1) but V1 is variable and mostly not near 300mV.

I use the LM324N quad DIP package from TI.

Does anyone have an idea where the problem might be?

Why have you chosen such low values for those resistors? I think you are asking the LM324 to sink too much current. The design in the main circuit diagram (with the multiplexer) loads both parts of the LM324 with 10 kΩ, not 1 kΩ as you have done, meaning that the difference amplifier has to sink only 1/10th of the current.

I cannot say what the maximum sink/source current is, because the data sheet only specifies it at VCC = 10 V. It will be much lower at VCC = 3,3 V.

Looking at the Texas Instruments data sheet, the LM324N is only guaranteed to sink 5 mA! (If I’ve read it correctly.)

Thank you for the reply! That was indeed the problem. The current what is going over the OP was definetly to high to sink that current with the 1k resistors. I’ve measured a current of 5.4mA.

Will have to go with a bit higher resistance than the 10k because there is in the higher resistance of my RTD also a small leak to the OP.

If you look at the data sheet examples, they use resistors in the 10 kΩ - 100kΩ range (though one example uses a feedback resistor of 10 MΩ), so 150 kΩ and 100 kΩ would probably be good values to try. You do not need the ratio to be 1.5 exactly, because you can scale the number in software.