PLC 5 Analog Input Noise Issues


Thread Starter


I have a problem on an Analog Input Card on a PLC5.

The signal on one channel increases the value on other channels.

The inputs have different power supplies. The first three channels have a 4-20mA signal from pressure transducers having same power supply.
I add another 4-20mA from a temperature sensor. Now when I switch on power on the temperature sensor the pressure values on the other channels increase????

Please help!!!
Adding the additional 4-20mA signal exceeded the common mode rejection ability of the analog input card.

You've created a ground loop by adding a signal whose reference ground is at a different potential than the original signals.

A mild ground loop appears as an offset or noise. A severe ground loop offset drives the signal offscale. A really bad ground loop fries the analog input.

Put a 'loop isolator' between the temperature sensor output and the analog input. A loop isolator has a 4-20mA input but isolates its 4-20mA output.
Analog inputs typically come in two types - non-isolated and isolated. Non-isolated inputs share ground/returns (typically in groups of 4 or 8 inputs), while isolated inputs are isolated from each other. For example, AB 1771-IFE is a non-isolated analog input card while a 1771-IL is an isolated analog input card. The 1771-IFE also has a limit that analog input signals must be within +14.25V referenced
to module common.

If you hooked up two different power supplies up a non-isolated card, it might be possible that this limit might be exceeded, which will cause an internal ground loop in the card and incorrect readings. Another problem can occur if the two power supplies do not have the same ground, which will create a ground loop if you ground both to the same common in the analog input card ,which can cause incorrect readings or sometimes some strange effects

It is a good engineering practice to always use isolated inputs.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP
713-909-2139 (o)
The FC-11 4-20 mA Isolated Signal Conditioner (at the link provided) will work in this situation.

I agree with Mr. Mostia, it's good practice to always use isolated inputs.

It also pays to use temperature transmitters that are isolated.

Lots of vendors have similar offerings.

I suspect that the offset problem will go away when you use a loop isolator on the temperature signal. Adding isolators to all channels is probably overkill.
Hello William,
You are right I have AB 1771-IFE . Now the question is
how to solve this problem. As David suggested I think I can just add a signal isolator /conditioner for the one input I need to add. Do you suggest any signal isolator? Replacing the card will be more expensive I think.

Thanks for your input.

I installed the isolation module to the added input and solved the problem.

Many thanks to David and William.