I've been lurking through these forums for years but just now had the urge to ask for help.
I'm creating a thermometer (using an NTC thermistor) for my college project. I began to analyze my sensor in terms of its linearity, temperature range, etc (specifications in general) ... For this project, I decided to use a temperature range from 55ºC to 150ºC (celsius).
Afterwards, I implemented my sensor in a wheatstone bridge. In order to do so, I defined my lowest temperature (55ºC) resistance value (2989 Ohms) and my highest temperature (150ºC) resistance value (182.6 Ohms). I proceeded to find the value of the other 3 resistances so I could end up with a [0 ---- 5] voltage at the end of my bridge (beeing 0V to 182.6 Ohms and 5V to 2989).
My biggest problem now is implementing an I.A.:
My current wheatstone bridge voltage ranges from -13mV to 5.05V which is fine (tho I'm currently designing a gain = 2x I.A. and I'll then throw it back to my voltage range by using a summing amplifier in order to reduce circuit's noise). Thing is, I'm not able to have 2x my bridge's voltage at the end of my I.A. (even tough I'm following the I.A. equation in order to find it's gain).
As you can see, I simply defined the same value for every resistance besides the middle one (this should mean that (1+(2*Any-other-resistance / Rmiddle)) * (V+ - V-) = 2(V+ - V-), which should define a voltage range of [-0,0026 ---- 10,1]V).
Note: I'm using TL084's to build my I.A. (tho I'm testing some other components like TL082 or LM324)
There's a picture attached as I know I messed up explaining this. Any help would be appreciated. I'm really stuck and I have no idea what's the problem (also thought it might be balancing the bridge or having high voltage on my op amps but atm I'm just throwing possibilities into the air without any thinking).