4-20mA from Load Cell Amplifier Noise Issues

Hello, I am developing a project where I have a load cell mV signal that needs to be amplified to a 4-20mA signal for a PLC to read. However, I need this project to be noise resistant.

When using a handheld 2 way radio next to the load cell amplifier, the mA output readings jump all over the place +/- up to a few mA.

I recently tested this board which says "The internal embedded active low pass filter can effectively filter electromagnetic interference in industrial site."

From a load cell simulator I have the E+ E- S+ S- and shield connected, and for the output I have the 24v supply powering it, and 2 wires connected to I0 (current out) and "com" in a shielded cable being where I get my 4-20mA reading. I also have one end of the shield wire connected to the same ground that is connected to the power supply I am using. Am I missing something or wiring the ground/shield wrong?

Everything works accordingly, except this issue with noise.

Any help with this problem would be greatly appreciated!

Thank you, Matt
Chasing noise in analog signals is always a daunting task. I found a good document on shielding at mouser.com (https://www.mouser.com/pdfdocs/alphawire-Understanding-Shielded-Cable.pdf) There are different types of shielded cables and some with twisted pair as well as shielded. Make sure you are using the best style of shield cable you can acquire. Also, make sure you are only connecting the drain wire or shield at one end, and avoid ground loops whenever possible. If all of that fails maybe run the cable through a shielded conduit or separate the cable from the radio frequency source as much as possible.

Good luck
Finding noise and solving these issues can be a daunting task. It is a matter of doing all the things as they should be emc-wise, so:
1) use Japanese electronics for the signal convertor. I have very very very good experience with Japanese electronics in combination with weak sensitive signals. Since Japan has a rock-soil they have to design electronics in such a way, that it works without earthing.
See for example this: https://www.kyowa-ei.com/eng/product/category/acquisition/wga-650b/index.html
Japanese electronics are way less susceptible to noise, and the quality is also often much higher.
Large benefit with Japanese electronics: no earth needed!

There is more that you can do of course, here is my experience of 25 years with sensitive signals:
2) use shielded/screened cables, preferably of twisted wires inside
3) use thicker wires (more resistance=creates more voltage with noise, less resistance=less noise voltage)
4) you have a DC signal, so earth the screen only to a real clean earth and only on the side of the measuring part, in this case, the signal to 4-20mA convertor. The 4-20mA signal coming out of the converter is always very robust, and this cable can be very long. Only for AC signals you have to connect the screen to clean earth on 2 sides, but you have a DC signal, so only earth to clean earth on the measuring side: the convertor.
5) put the convertor as close to the sensor as possible.
6) use a metal box around the convertor, preferably a real EMC enclosure where the sides/door are well connected to each other. This box will keep signals coming from outside through the air out. Maybe first try with aluminium foil around the convertor, connected provisionally to earth (to try).
7) ground this box properly to a clean earth
8) use EMC cable glands (so not the standard ones) that connect the screen around the cable going into the box in a good way fully around the signals. Dont "screw" the screen into a small wire to connect: you then create a 100uH coil and the noise signal doesn't want to go through there to the clean earth.
9) if you don't have a clean earth, make one: drive a copper pipe in the ground as deep as possible. If the soil is dry: water it every day. In case of emergency I once drove a pin in the ground of a very dirty sewer system (with lots of rats inside). Look out with cable isolation, the rats like to eat the isolation
10) measure AC voltage between the neutral and PE of wall sockets: this should be just a few 10th of mV. If not: make a (provisional) clean earth with the pin
11) Maybe the noise comes in through your power supply! Use another power supply and/or put the power supply also in the metal box. Use screened cables with the EMC cable glands.