AI module confusion

[yfcarer]

Hi, All. I met a problem when I do the test for a control cabinet. It is when I test the AI module(1769-IF8).

My site signal is 4~20mA, which comes from a LED indicator. The indicator is already powered through power supply cable. As indicated in the manual, I wire the 4~20mA input terminal to the "I in +" and "V/I In -" terminal of the AI module.

After that, I use the AO module(1769-OF8C) to generate a 4~20mA signal and connect it to the AI input terminal. Then monitor the raw value form RSlogix5000. The reading is steady with a +-0.25% error.

Then I use a signal generator to generate a 4~20mA signal and wire it to the 4~20mA signal terminal. Strange thing happens. The reading still has an error, but the reading is unsteady , it shakes from -0.4%~+0.4%.

Did I make some mistake when wiring? Or what the difference between a 4~20mA signal generated from AO module and a signal from the signal generator?

Has anybody met such condition?
Any suggestion is welcomed.
Thanks in advance!!!

 

[keithkyll]

The signal generator is powered from the AC line (mains).
The ground is NOT common between the 2 units. The unsteady reading is caused by noise on the grounds.

The only difference between an Current Analog module and a Voltage module is 1 resistor. Connect a 250 ohm resistor across the input, and now it's a Current Module. 4 ma across 250 ohms equals 1 volt. (some use 500 ohms and 0-10/2-10 volts).
Connect a DC supply to the input. 1 volt is 4 ma, 5 volts is 20 ma.
Another way is to connect a 1.5 volt battery across the input. 1.5 volts is 6ma. Monitor actual battery voltage while connected.