Dear friends,
I am working on reading 4-20 mA from Level/pressure/flow transmitters. I used to use a 10 ohm resistor in series. so I measured the voltage drop on the resistor but in this circuit I needed the ground from the transmitter device. Is there a better method not using ground wire from the transmitter?
I see that if one applies differential inputs out of VCC-GND range of the op-amp then it does not work as expected so if you say I measure differential inputs but dont know the voltage of each with respect to GND then how can op-amp work!
Sorry, but I do not understand you.
An OP-amp has two inputs and can work with differential data. But both the inputs and the output(s) needs to be slightly inside the supply range of the OP-amp.
So the cost/complexity of your design will increase if you need full isolation.
One solution is to use a linear optocoupler where the current is used to modulate the LED.
Another is to have a voltage-controlled-oscillator feeding a transformer and pick up the frequency on the other winding.
A third is to send the current through a coil and use a Hall effect sensor to measure the strength of the produced magnetic field.
Yet another option is to isolate your measurement device - then you can send the current through a resistor and directly measure the voltage. Then you use whatever isolation you need in the next step - forwarding the measured value. Power supplies are isolated, just as 10base-T, 100base-T, ... Ethernet connectors, making it easy with a single input forwarded over Ethernet.
Or open the wallet and spend some money on something like: www.fine-tek.com/.../pinfo_in.aspx