Power supply ground isolation

[vikash23]

Hi,

I am trying to isolate my battery ground with RF and ADC ground by an inductor as shown in the figure. Example diagram shown

Can I please know if this way of isolation is fine or will it create any problem ?

You can see the inductor part number and please do let me know if I can use this part.

https://obrazki.elektroda.pl/9856117500_1380799404.jpg

 

[chuckey]

For electronic circuits it is best to have a common ground, this is because if the grounds have different voltages on them, then if anything changes (external coax braid is earthed for instance) what happens to the amplifier is not known, will it oscillate, blow a fuse, suffer a change in frequency response.
The only time separate earths are used is in places like TV studios where you need a mains earth for safely earthing external metal work of equipment and low level technical equipment which has its own earth circuit which is tied to the mains earth via a 47 ohm resistor. This is because in a large studio complex it is difficult to decide what is earth and what is earth 200m away and the technical earth is not rated to carry the fusing currents of large mains powered air conditioner units and the such like.
Frank

 

[ftsolutions]

You could use that part, though I don't know what else may be drawing power from the battery source, and the voltage regulator you show in the schematic is good for 200mA, so that will completely max out the bead inductor you show with no other current returns. You may need to use a higher rated part. As I have no idea about the frequency content of your RF circuitry, I cannot say that the frequency response of the part will be good or not. Murata does offer sample kits for some of these parts, so you could get a selection to try. Sometimes one has to also consider a bead on the +V supply line to match the reference GND line filter as well.

I've used similar parts in various embedded control systems to keep high energy switching noise out of sensitive A/D reference nodes, etc.

 

[vikash23]

Hi ftsolutions,

Thank you for your reply.

I was not able to find sample kits from Muarata. they offer only free sample parts.

The frequency content of the RF circuitry id 2.4GHz. I have used the same ferrite bead near to the RF +ve supply.

Can I please know will there be any difference if I move the inductor next to the pin number 2 of the U11.

What is the beast method selecting a inductor depending upon the impedance. what effect will that cause if high/ low impedance inductor is selected in a circuit ?

 

[ftsolutions]

I think that you can still get the ferrite kit - though mine is several years old it does have a sampling of SMT0603, SMT1206, and some SMT1806 parts of various materials. I do not know where you are located, but an electronics distributor here in the US does carry several of these types of kits - go to this webpage if you can order:
https://www.newark.com/ferrite-assortments
The part number of the kit I have is EKEMBL18E. Unfortunately it does not contain your indicated part, BLM21AG121SN1D. You might find it, or a different one, to be useful on your projects. your chosen part, I see, it rated for up to 800mA.

There may be some difference in effectiveness, depending upon physically where you place the inductor/ferrite bead. I would not put it where the 3.3V RF gnd current must pass through the bead to return to the regulator, but placing the bead just beyond that point to where it then goes out to the battery or other GND reference could be fine, depending upon the physical layout of the circuits. It if there is additional coupling in the printed circuit assembly and the traces are long to the battery gnd connection, it might be better to locate the bead closer to the battery connection. Looking at your schematic, you *might* want to consider possibility of moving the inductor to the other side of your input capacitor, so that the HF impedance does not interfere with the regulator's unit step-response. This would alter the circuit though, in that the input capacitor would also be grounded to the RF gnd, and not the battery ground. It may work OK for you as you have shown.

Do you know how much 2.4GHz (and sub-harmonics) interference you will see? Sometimes it is hard to accurately calculate in advance. Without knowing all the details of your design, I would say that your choice of bead is a reasonable starting point. I'd recommend that you get a few higher impedance ones, and maybe a lower impedance one to try also, maybe a 220 Ohm and 600 Ohm, and perhaps something like a 47 ohm or 10 ohm so you can cover a broad range. These parts do not exhibit a high 'Q' factor, as opposed to the BLMxxAB series parts (which for your situation may be a good thing) so there will likely be some overlap in effectiveness, but ideally you want to balance your selection with sufficient filtering of the RF with the lowest necessary resistance. Do you have a particular emission standards to meet? Is the case/chassis grounded directly to the battery ground? If the filtering is going to be critical, you may need to use high impedance FET probes on your oscilloscope to measure the differences obtained with the beads, as the standard oscilloscope probes will be too low impedance at higher frequencies to show you accurately.

 

[LISA_DALTON]

This circuit does not isolate the ground at all. They are still tied together.

 

[ftsolutions]

I am working under the assumption that the originator's intent is to provide noise isolation/reduction to protect a sensitive ground net and not achieve true galvanic isolation, based upon the description and original circuit. As there are many on this forum with varying capabilities in the English language, I try not to get caught up in strict semantics unless it is critical to resolution of the problem. This assumption might be incorrect in this case, but I don't think so.

 

[LISA_DALTON]

I believe you must be correct, because it is just too obvious that it is not galvanically isolated. I was not trying to play semantics, but I do plead guilty to maybe jumping the gun on my response. My error.

 

[Jibby Benjamen]

Yes, you can use.