SodBuster493 (Customer) asked a question.
Noisy RTD Noise
Hey all, I seem to be having an issue with a noisy signal on my RTDs. I am using a productivity 1000, and the P1-04RTD module, and am using 4-wire RTDs, in all 4 slots of the module. But I'm getting a lot of noise on the reading and I'm having a hard time troubleshooting. I'm getting a noise variation of about 0.7 deg C. It's relatively minor, and can filter it out in the ladder if I need to, but I'd like to see if I can fix the source first. Some details:
- RTDs are PT100, class A
- Module filter set to 61ms
- Probes are connected via M12 cable
- 4 wires (R1+, R1-, R2+, R2-) feed into a terminal block.
- All wires to the terminal blocks are the same length, and all wires from the terminal block are the same length.
- R1+ and R1- are connected to the module R+ and R- terminals of the PLC module
- R2+ is left un-wired and R2- is wired to 0V
- Both COM terminals of the PLC module are wired to 0V
- AC power is in the vicinity, but no high power or inductive loads in proximity.
- I have tried ferrite cores to minimize the noise, with no change.
- Testing with a multimeter, I can see about a 0-22mv jump in the signal that pulses at a fairly regular at 1Hz.
What I'm doing wrong?
https://www.electronicdesign.com/technologies/analog/article/21796022/improve-noise-immunity-on-rtd-ratiometric-measurements
The above article may help you out. Capacitors are used to filter out the noise.
Regards,
Garry
https://accautomation.ca/
Thank you Garry,
That's actually a little reassuring that I wasn't doing something crazy wrong. A closer look at the cable I'm using shows that it's not shielded, despite being marketed as an RTD cable. I'll try switching out the cables and see if I get an improvement. If that doesn't work, I'll weigh the cost of filters vs smoothing it in the ladder logic.
-Nate