Gents,

I would really appreciate your help on this. A system already exists that takes a 4 to 20 ma pressure transmitter output, most likely changes from current to voltage and goes into a microcontroller. What i would like to do is tap into the line ( a T connection) and measure the signal with a DAQ device (I will use DATAQ DI-155). The DI-155 measures only voltage so I plan to use a 250 ohm resistor to achieve a 1-5v output. My question is, if i do this, will i change the output signal from transmitter because im introducing the 250 ohm resistor to the circuit. What is the best approach for me to take this measurement?

 

If you have a true current source AND it has a compliance of at least 5V it should be OK. By compliance, I mean that the current driver has to apply some voltage to get current through the various resistances in the loop while preserving accuracy. It needs to be able to drive to 5V to get 20ma through the resistor you propose. For this reason, most current shunts add a only small resistance to the loop thus requiring a smaller compliance voltage. If you need more volts to measure, use a differential amp.

Depending on your ADC, you could use smaller shunt voltage directly by seting Vref to the top range of this smaller voltage. Then the ADC range would be 0-maxCount over that voltage, preserving resolution.

 

Ok Thanks. I am using above 20V so this sounds like it should work. I will try it out.