Do I need to put a ferrite bead near a RS485 receiver device power pin?

Question

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:

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:

MAX device with no ferrite bead:

Encoders connector:

Answer 1

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.

Answer 2

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).

Answer 3

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.