Alright. I will try and explain this the best I can in words. I have a 2-wire pressure sensor for a water holding tank that outputs an analog signal in the industry standard 4-20ma. It is connected to a VFD that controls the water pump and is giving it a voltage source of ~24v. I need to use an arduino to gather the information off of the 2 wire pressure sensor without compromising or having to completely reprogram the VFD. I have found some data on using a 250ohm resistor to give me 1-5v to the arduino, but I'm fairly new to all of this and it seems like that would screw up the VFD's data. Any help would be appreciated.
4-20 is a current signal. any resistor you place in-line will have a voltage drop across it.
firstly, ohm out the leads on the VFD. if it reads 250 ohms, you have the answer. just measure voltage across these pins.
as you know, you will get a 1-5V signal or 200 to 1000 on the Analoge in.
A 4-20ma duplicator might be usefull in your situation, correct me if I'm wrong but you want the arduino to gather info without effecting the signal to the VFD from the pressure sensor, also does the vfd have a current loop output? ( I ask as some do)
I have been playing around with ad694 - could be of some use to you
http://www.analog.com/static/imported-files/data_sheets/AD694.pdf