I have been doing research for the past week on setting up a 100 Ohm Platinum RTD to work with labjack U3 LV (I have to use labjack). I know I have to start off with a wheatstone bridge, and use a three wire connection since that is how many wires each RTD has.
For reference it is the labjack U3 LV. I could not post the link as the post would not let me.
This is what I thought the configuration would roughly look like. However I do not know if I need to add a differential amplifier, or an instrumentation-amplifier to obtain a good signal to labjack?
Below you'll see the typical InAmp with 3 op amps that I thought I would have to set up with my wheatstone bridge circuit.
However I don't even know how to begin choosing resistors for this, or how to properly set it up with the wheatstone bridge. The question remainds should I use this InAmp?
If neither of these would be suitable to obtain good readings from the RTDs then how should I set up my circuit? I am not very well versed in setting up circuits and much less using op amps. So if there is an alternative could you explain what it is, and why it should be used and how would I go about finding the resistors I would have to use for it to output a good reading, and the current that would run through it to obtain a good reading on the labjack? Actually I believe the current is 1 mA for most of the PT100's? Unfortunately I do not have the exact data sheets as this system has already been set up as is for years without use.
The version of the labjack that I have can handle 0 – 2.4 V and 0 – 3.6 V. (12 bit) Currently I am also not sure how I would power it, but I was thinking of using a DC Source like a small battery. I'd like to verify that I would need a voltage divider, and if so how would I go about incorporating it into the circuit?
I have been attempting to understand this set up, but the op amp part is what is confusing me the most. Since I know how to balance a normal wheatstone bridge, is there anything different I would do for this? Overall I'll be getting readings from 8 RTDs, but I figure once I figure out how to do one of them I can just do the rest of them the same way since they are all the same. For reference these RTDs are on heat pumps and will eventually be used in a GUI that will convert the voltage received on labjack to a temperature. All help is appreciated! Thanks!
Best Answer
Instrumentation amplifier is rather expensive. The input impedance is very huge, therefore it won't influence the wheatstone bridge. Further, what a RTD conditioner has to have is a low pass filter at input stage. You can place a RC filter on input, that again won't influence the bridge, since the input impedance of IA is unlimited. But there are ready made ADCs for RTD like ADS1248,...that are cheaper than a single IA.