Analog readings unstable

I have pt100 sensor connected to fez spider analog input. The resistance of sensor changes, along with temperature. I have 110 ohm resistor connected in series, so i am meassuring voltage drops. The problem is, that my readings are inconstitent , from 20 to 30C. Like that some kind of noise is present. I am just starter in electronic, so i had no idea, what causes this voltage up and downs. Is the 3.3v voltage reference ?


Do you have a link for your sensor?

Yes, description of sensor is here .

How often are you measuring? There could be some noise too so maybe add a smoothing capacitor to reduce it. Got a pic of your setup??

also ground the inputs of the unused analog inputs.

Unfortunately, i have not spider at the moment. My readings are every second. Sensor is connected to p4 into analog input…

My unused inputs are not grounded. I will ground them, and report results when i get spider back.

Thanks for tip

Don’t know, if this is right way, but :

  • pt100 have variable resistance, what I need is voltage.
  • I need some known resistance, to calculate voltage drop
  • I add resitor with 110 ohms in series, and use this formula:

I = ReadedVoltage / 110 ohm
R = (3.3V - ReadedVoltage) / I
temperature = GetPTTemp (R) 

I was testing in cca 25C, but temperature oscilates from 20C - 30C. The sensor is under fez voltage. It’s 2 wire sensor. If i got everytime same ReadedVoltage in same temperature environment, but temerature was wrong, i would point at my code that calculates temperature from resistance. But the problem is, that ReadedVoltage oscilates. I’m using ReadVoltage() method of AnalogInput class.

Thanks has some interesting schematics and equations. Do you know what accuracy your PT100 is meant to be?

Thank you all for informations, but this already exceeds my current electronic skills. If I just simplify all just to function ReadVoltage() that varies - I suppose I need something like lm317?

Thanks, for now I got enough information. Now I have to study solution.

DS18B20’s are (relatively) cheap and easy to use as an alternative

@ Andre, yes, true, but again nobody knows what circumstances this is being tried under except for @ tvinko. If this is just a home project with no specific requirements then an alternative like this is very suitable - @ tvinko already admitted the complexity discussed was above their level of electronics knowledge, so it may only have been a choice of a device that seemed easy (but is in fact not quite as easy) that arrived at the PT100… Just saying.

Yes, this is just home project. More like learning electronics, hopefully with no deadlines :slight_smile:
Maybe i choose too complex sensor for starting with…