Where to put ferrite bead at analog signal transmission line - Page 1
Forum section: electronics / beginners

author: müllerbua
author_rank: Newbie
timestamp: April 16, 2020, 08:49:51 am
content: Hello everybody!I have a question regarding EMI filtering in analog signal lines.I am designing a signal transmission which transfers analog measurment signals from an power inverter circuit. The signal is in the range -10-10V and the frequency around 100-300Hz. The signals are transferred via Ethernet cables. In order to reduce and filter EMI noise, ferrite beads should be placed at the end of the Ethernet cable.My question is: is it also necessary to put ferrite beads at the starting side of the Ethernet cable, where the signal enters? I couldn't find information regarding this or if it has effects on the signal.Thank you and kind regards,Christoph

author: David Hess
author_rank: Super Contributor
timestamp: April 16, 2020, 11:22:01 am
content: Quote from: müllerbua on April 16, 2020, 08:49:51 amIt can be. Usually people think of conducted RFI getting into the receiver side of a cable but sometimes the transmitter side can be affected. A good example of this is audio power amplifiers which require RFI filtering on their *output* to prevent problems.

author: müllerbua
author_rank: Newbie
timestamp: April 16, 2020, 11:32:02 am
content: Thank you for the quick response! I understand!The signal on transmitter side is injected by an isolating amplifier. So I suppose this is enough to cancel out the RFI?

author: RoGeorge
author_rank: Super Contributor
timestamp: April 16, 2020, 12:00:03 pm
content: Think about it like this: the ferrite beads (in case of EMI filters) act against noises as a mirror acts against light.While this is not pedantically correct, one can say "EMI filters are reflecting the noises back from where they came from".Example 1: If you want to prevent outside noises to enter into a sensitive amplifier, the filter must be placed near the input of the amplifier.Example 2: If you want to prevent the noises produced by a switching power to go out and pollute the outside world with its spurious RF, then the EMI filter must be placed right at the output of the switching power supply.

author: pwlps
author_rank: Frequent Contributor
timestamp: April 16, 2020, 04:23:43 pm
content: Quote from: müllerbua on April 16, 2020, 11:32:02 amA clamp-on ferrite bead placed on the Ethernet cable will act as a balun i.e. it will block common-mode RF. This is enough to block inductively generated parasitic EMI (without limiting the signal bandwidth) because the signals are transmitted by twisted pairs so that induction can only generate common-mode voltages. If your isolation amplifier outputs a purely differential signal and is insensitive to any incoming (or reflected) common-mode voltage then I don't see any reason a priori to put a balun on the output side. Interesting question though.

author: Marco
author_rank: Super Contributor
timestamp: April 16, 2020, 05:22:17 pm
content: Disturbance in the frequency of interest is irrelevant any way. This is not some high impedance microphone or guitar coil pickup, at 100-300Hz if you have a source capable of impressing any significant signal on the cable which can win from your low impedance driver you've got bigger priorities ... like duck and cover.

author: David Hess
author_rank: Super Contributor
timestamp: April 16, 2020, 06:12:21 pm
content: Quote from: müllerbua on April 16, 2020, 11:32:02 amThat can keep the RFI out of the rest of the circuits but the situation I described was where the RFI affects the output stage of the transmitter itself.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 16, 2020, 06:22:24 pm
content: If the bandwidth is truly ~300Hz then it doesn't matter, ferrite beads do absolutely nothing down there.They are useful if the bandwidth is not truly so low, however. Your receiver might be designed with an R+C filter at its input, but perhaps it is poorly grounded, and common mode noise introduces errors. Or the filter isn't well constructed, or doesn't have enough attenuation at high frequencies, and some HF is let through, which causes rectification against semiconductor junctions (typically a problem with bipolar op-amps, but can happen anywhere given enough intensity). Or the same affects the transmitter, or its feedback/sense pin. Or the transmitter isn't stable into the cable impedance and oscillates.So, it's a good general approach to add RC or LC lowpass filters, to attenuate outside noise; R||L and R+C networks, to stabilize line impedance; use common mode chokes; and etc. (When and where to use them, and which, is of course the challenge.) Only use the bandwidth you need!Tim

author: müllerbua
author_rank: Newbie
timestamp: April 17, 2020, 01:52:04 pm
content: Thanks for all your answers! I appreciate it a lot and already understand more about using them!