Same physics as David’s speaker, yes. The RF burst gets rectified by the input stage of the ADC (or any semiconductor junction on the line), and that shows up as a DC offset for the length of the burst. 2G-style bursts at 217 Hz are the worst, but LTE uplink bursts do it too. A cabinet I look after at work had exactly your picture, spikes during uploads only.
What fixed ours, cheapest first:
- Get the antenna out of the cabinet. A short-cable antenna on the cabinet roof. Inside a steel box the router is shouting at your wiring from 30 cm.
- Shielded twisted pair for the 4-20 mA run, shield grounded at the controller end only.
- An RC filter right at the ADC pin, 1 k and 100 nF, plus a 100 pF cap close to the pin for the RF itself. Your level doesn’t change in milliseconds, you can afford it.
- A clip-on ferrite on the signal pair where it enters the controller.
- If the router has a setting to lock it to LTE and not fall back to 2G, use it.
After the first two the spikes were gone for us. Old spikes stay in your history, so if anyone set an alarm on that level, check whether they were tripping it. A readings-in-a-row debounce of 2 or 3 on the high-level rule hides any single leftover.
QoS 1 and a lot of patience.