Dear all,
until recently I was confident in my filtering practices, however now I would like to put my rigour to the test and see what the community thinks.
I have attached an image of RFI filtering I added onto some differential signals. I shall in point form outline my line of thinking and aim to discuss each point.
Let us focus on SIN+ and SIN- differential signals coming IN to the circuit from pins 14 and 7 at the left of the image.
1. C204 and C205 are X type 0603 package capacitors. The intent is to redirect RFI noise to the metal chassis/heatsink to avoid reflecting incoming noise back out and to avoid transmitting internally generated noise out.
2. FBP26 (Ferrite bead pack) is a common mode choke so will present a high impedance to common mode noise/signals and the aim there is to try and get the differential/common mode noise through the caps instead. The caps would also help to get rid of the differential noise which might pass through the common mode chokes easily.
Questions from me:
1. The 0603 package leave ~0.6mm of clearance between the signal pad and RF GND pad (i.e. chassis which is bonded to earth). Typically I maintain a design rule to keep 1mm of clearance from signals to earth tracks, but in these small components lies an exception. Is there a risk of a well bonded chassis leaking noise back in through capacitors of this nature not only through the capacitance, but also through this smaller clearance? I am assuming a well bonded earth with lots of surrounding heavy duty metal.
2. Whether the ferrites are common mode chokes or just ferrites, my idea was to try and force the noise out through the caps. Now I wonder if it isn't better to have the ferrites in front of the caps to try and eat up the noise and dissipate it as heat before it gets round to the caps. This however would reverse the situation for the internally generated noise but I assume in most cases that long cable runs result in much more RF type noise than internal switching circuits @ ~100MHz? This concept also applies to replacing the ferrite with a small resistor and forming an RC network where the R is intended to help dissipate some of the noise as heat rather than blasting it all through to the chassis.
Please keep in mind that this is only signal filtering so minimal components is desirable (hence me limiting myself to just one cap and one ferrite).
Please also keep in mind that the philosophy described in this post is already in my design so I am very hesitant to change it unless you all seem to think it is an absolutely god awful way to go about filtering.
I look forward to your responses of your own best practices and what you have/have not had good experiences with.
Kind Regards,
James
until recently I was confident in my filtering practices, however now I would like to put my rigour to the test and see what the community thinks.
I have attached an image of RFI filtering I added onto some differential signals. I shall in point form outline my line of thinking and aim to discuss each point.
Let us focus on SIN+ and SIN- differential signals coming IN to the circuit from pins 14 and 7 at the left of the image.
1. C204 and C205 are X type 0603 package capacitors. The intent is to redirect RFI noise to the metal chassis/heatsink to avoid reflecting incoming noise back out and to avoid transmitting internally generated noise out.
2. FBP26 (Ferrite bead pack) is a common mode choke so will present a high impedance to common mode noise/signals and the aim there is to try and get the differential/common mode noise through the caps instead. The caps would also help to get rid of the differential noise which might pass through the common mode chokes easily.
Questions from me:
1. The 0603 package leave ~0.6mm of clearance between the signal pad and RF GND pad (i.e. chassis which is bonded to earth). Typically I maintain a design rule to keep 1mm of clearance from signals to earth tracks, but in these small components lies an exception. Is there a risk of a well bonded chassis leaking noise back in through capacitors of this nature not only through the capacitance, but also through this smaller clearance? I am assuming a well bonded earth with lots of surrounding heavy duty metal.
2. Whether the ferrites are common mode chokes or just ferrites, my idea was to try and force the noise out through the caps. Now I wonder if it isn't better to have the ferrites in front of the caps to try and eat up the noise and dissipate it as heat before it gets round to the caps. This however would reverse the situation for the internally generated noise but I assume in most cases that long cable runs result in much more RF type noise than internal switching circuits @ ~100MHz? This concept also applies to replacing the ferrite with a small resistor and forming an RC network where the R is intended to help dissipate some of the noise as heat rather than blasting it all through to the chassis.
Please keep in mind that this is only signal filtering so minimal components is desirable (hence me limiting myself to just one cap and one ferrite).
Please also keep in mind that the philosophy described in this post is already in my design so I am very hesitant to change it unless you all seem to think it is an absolutely god awful way to go about filtering.
I look forward to your responses of your own best practices and what you have/have not had good experiences with.
Kind Regards,
James
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