ISO3082: ISO485 Radiated radio-frequency electromagnetic field immunity test

Hi,

I am sorry hear that you are facing issues in meeting Radiated Immunity requirement.
Could you please let me know what standard are you trying to comply? IEC61000-4-3 requires the testing to be conducted for frequencies >80MHz. Could you please confirm?
Could you please also let us know what radiation levels are you trying to meet? Like is it 10V/m?

Radiations can be picked up by various components on the PCB like the magnetics, traces and others. A good practice make a system immune to such noise is to bandwidth limit all the inputs of a device using RC filters such that datarates are not affected. I am not sure if the communication data is corrupted at the CPU, Side1 of ISO3082 or on the RS-485 bus. This has to be found out by experiments.

The RS-485 bus is differential and is generally immune to common-mode noise. It is also worthwhile to remove the common-mode choke L10 on RS-485 bus and test to see if that makes any difference. Let me know, thanks.


Regards,
Koteshwar Rao

my device requires the testing to be conducted for frequencies from 0~80M,the radiation levels is 10V/m.By experiments, we found 485 receive and transmit are not normal at 2.7M OR 3.7M

You are not nice, you don’t answer the questions I sent.Finally, after an experiment, it was found that a 120 ohm resistor was connected to the terminal, and the problem was solved.So I want to ask a question, is there a chip with a terminal impedance of 120 ohms and compatible with the ISO3082 chip?

Hi,

I am very sorry that I couldn't come back to you with more inputs to resolve the issue you were facing and I am glad to know that you could meet the radiated immunity requirement by placing a termination resistor of 120Ω on RS-485 bus.

Regarding your question on 120Ω terminal impedance, did you mean to say that you are looking for a device with 120Ω of terminal impedance integrated? Terminal resistors are typically not integrated into devices and they are placement on the bus is quite dependent on connection topology.

Please go through the below two App Notes that describe on RS-485 interface in general and also talk about termination resistor placement.

www.ti.com/.../litabsmultiplefilelist.tsp
www.ti.com/lit/an/slla272c/slla272c.pdf

Let me know if you have any other questions and I apologize that I couldn't provide you with more inputs to address the issue. Thank you.


Regards,
Koteshwar Rao