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I'm using a MAX3094ECSE+ RS-422/485 receiver receiver datasheet for an application that is receiving quadrature encoder wires. Those wires are power, positive and negative signals from quadrature external encoder. Encoder outputs are differential signals: A+, A-, B+, B-. They need, on the pcb, to be converted into A and B signals that will be connected to STM32F207ZG timer channel pins.

So I have used MAX3094ECSE for doing this. This was working fine on my protoboard prototypes, powered by a stable power source. But now I am facing the pcb design circuit. And now I was also wondering for the convenience or not of using a ferrite bead between my power circuit and this kind of device.

I'm not an expert using ferrite bead. But I know that it is often used for a better EMI isolation between analog and digital circuit, so I put ferrites between analog GND plane and digital GND plane. I know that ferrites are used in series with power leads when connected to high speed ICs, in ex: microcontrollers. As higher speed, a bypass capacitor should be also needed:

analog and digital part connection

I don't know if quadrature encoder signals could be considered digital high speed signal or not. Furthermore I have to take into account the receiver output is going to microcontroller inputs. I'm not sure what kind of consideration I should have for the MAX receiver device, regarding this topic. Maybe the device needs it or maybe not.

My question:

Should I put ferrite bead between power 5V node and this Integrated Circuit Vcc pin? Could it work correctly without it here?

Could someone give me an answer to my question? if not, I hope to get his point of view.

NOTE: It is true that vendor doesn't suggest the use of ferrites with this device.

My circuit:

5V-power that powers 3 encoder and 2 MAX devices:

5V-power that powers 3 encoder and 2 MAX devices

MAX device with no ferrite bead:

MAX device with no ferrite bead

Encoders connector:

Encoders connector

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    \$\begingroup\$ I don't think there is enough info to answer if you should or should not put a ferrite bead. Even if it works without it, another thing is if it will pass EMI tests without it, or even with it. \$\endgroup\$
    – Justme
    Jul 24, 2023 at 11:49
  • \$\begingroup\$ Thanks, I don't know what kind of info does I need for answering this. I will still puting the ferrite here, so in case that it will be no needed I will put 0 Ohm resistors instead. Currently I'm unable to answer this. So I will make that, hoping I can find the solution as soon as possible. This product doesn't have to pass any EMI test, but I don't want the device be a source of strong interference or noise that makes this application working wrong. \$\endgroup\$
    – Suvi_Eu
    Jul 24, 2023 at 12:00

3 Answers 3

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Ferrite bead address any EMI issue and more likely to pass emc screening and save time. It reduce interference with other equipment within a room.

Electronicly it subjective especially digital signal unless there specific need. For example cut down noise emissions from rs485 to wideband analogue circuit. Pi circuit is good but has to justify extra cost and quality of board. If few board the yes fit in pi if there room in the circuit layout. In case not needed just modify bom and replace with a link or 0R smd.

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  • \$\begingroup\$ thanks for your explanation. It is helpful. When you say Pi circuit are you talking about two capacitor and one ferrite connected in Pi mode? Dou you think that adding two capacitor to the BOM could adding a great extra cost? The analogic part of my pcb is the power circuit. \$\endgroup\$
    – Suvi_Eu
    Jul 24, 2023 at 12:36
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No.

You certainly need a bypass capacitor local to the IC, after the ferrite bead.

With that addressed, the question is then: would the filtering help?

I suppose it might, if transmitters were present (large and intermittent current draw), and the main supply had to remain very quiet (which seems like an odd requirement in a mostly digital circuit). In which case, we need to know how quiet, what the impedances are, and design a filter accordingly (a ferrite bead is a good way to filter small load currents (up to 50-200mA depending on chip size*) at low attenuation (6-20dB).

There is no information suggesting this filtering is necessary or desirable, so I would be inclined to skip it.

*Ferrite bead inductance depends on DC bias current. Typical chips say 0603 to 1206 and 30-1000 ohms (at 100MHz) have a -30% saturation point in the 20 to 200mA range, with lower impedance types going a bit over an ampere. When higher currents are required, just use an inductor, and add damping resistance elsewhere (typically as a lossy capacitor).

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  • \$\begingroup\$ I agree there is no information suggesting this is needed. \$\endgroup\$
    – Suvi_Eu
    Jul 25, 2023 at 5:43
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Maybe! Honestly, people place them anywhere and don't really know if they are required or not. In your application, it might do a difference if the lines are very very noisy, but I have my doubts. The safest way to go is to leave a short footprint that you can cut later on if needed (you probably won't need it). EMI testing and on site testing will give you the answer if you need it or not.

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  • \$\begingroup\$ I also have my doubts, if the data signals received are noisy, what would it help to put a ferrite on receiver supply? The data would be just as noisy. \$\endgroup\$
    – Justme
    Jul 24, 2023 at 20:26
  • \$\begingroup\$ Well, comm can go trough while you generate way too much noise. Ferrite or typically placed at the power entry to keep the noise of the board on the board. If your power line is too noisy, ferrite won't do much! \$\endgroup\$
    – Julien
    Jul 24, 2023 at 20:31
  • \$\begingroup\$ A good designer should design his power entry by specifying a voltage range then placing a power supply with sufficient margin to allow for noise (depending on your environment). He will keep the ferrite if he can't use any regulator. Usually, those instance are for small sensors that are spaced constraints or things like that. That doesn't occur often. The other usage is to split ground planes which isn't the case here. The final reason to put a ferrite is to pass EMI. This is where my MAYBE come from! \$\endgroup\$
    – Julien
    Jul 24, 2023 at 20:36

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