Acquiring a current signal with FP-AI-112

[Robst]

Hi:

I need to acquire the output of a thermohigrometer, the output is 4-20 mA for both signals, temperature and %RH. I want to know if I can acquire the signal using and existent FP-AI-112 module adding a resistor between the input terminals of the module or if it is better to get a FP-AI-111 which is the ideal module for this application (I know). I guess adding the resistor may work, but I'm worried about possible damage to the 112. May this be harmful for the module?

Thanks for your reply.

Best regards.

Robst.

[Otis]

Hi Robst,

While you can use the FP-AI-112 to do Current Input, you must keep in mind that it's not quite what it's designed for.  But you can certainly do it (as this KB says).  Just place a 249 Ohm resistor (or other precision resistor that will not cause the voltage to exceed the desired range).  That will cause your signal to be between 0-5V. 

The only thing you might want to look out for is if the device has a chance of causing some ground loops between your FieldPoint and the device you are measuring.  The FP-AI-111 would be a better choice, because it will take differential measurements across a resistor instead of giving you the chance to fry your device.

In short, using the FP-AI-112 should work, but you'll be risking damaging the device.  If it's possible I would recommend getting the Current Input module.  But if money and time are really huge issues then take the risk and use what you've got.

Regards,