Chassis ground and Digital Ground connection using different components - Page 1
Forum section: electronics / beginners

author: Freshman
author_rank: Regular Contributor
timestamp: March 20, 2025, 03:36:24 am
content: In this reference design schematic, (https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/BoardDesignFiles/9303_sch.pdf) the chassis ground is connected to digital ground using R22. A couple of questions on this.1. What is the reason for having R22 and why it should be 1210 package and not 0603 or 0402?2. Can I replace R22 using a capacitor (for connection between the chassis and digital ground)? What would the impact difference between using R and C?Any typical capacitor value suggestion also will be helpful?

author: golden_labels
author_rank: Super Contributor
timestamp: March 20, 2025, 05:09:14 am
content: Quote from: Freshman on March 20, 2025, 03:36:24 amIt’s adirect link, not a resistor. They are just connected together.Quote from: Freshman on March 20, 2025, 03:36:24 amProbably no reason. Many components in this schematic seem overspecified. For example R15, R21, and R30 are specified as 1% resistors, despite they only limit current to a LED. It’s possible they simply provide values, which they tested to be working.Quote from: Freshman on March 20, 2025, 03:36:24 amExplain your reasoning. How did you come to the conclusion, that you need to, or even can, replace one component with another of completely different nature? One could as well ask, what happens, if you replace R22 with a banana peel, an it would make more sense.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 20, 2025, 12:25:45 pm
content: Quote from: Freshman on March 20, 2025, 03:36:24 amIt's very likely that the designer was wary of EMC issues with the design, which can sometimes be influenced by the means of connecting the circuit GND to chassis. Therefore they put a component in series with that path, which would allow them to easily change it depending on EMC testing results (sometimes a short works best, sometimes an open, sometimes a capacitor, sometimes a ferrite bead, etc). It's likely that they found that a direct short worked fine, so in the final rev they changed it to a 0 ohm resistor.If you're thinking about using this as the basis for your own designs, I would also encourage you to leave yourself a great deal of flexibility. For example a larger package lets allows more options for components to use (and makes the reworks a lot easier). I sometimes also leave myself multiple options for where this component is on the board (meaning on the schematic there are multiple of these components in parallel).

author: Freshman
author_rank: Regular Contributor
timestamp: March 20, 2025, 01:58:06 pm
content: Can you just shed some light on how EMC impacts if circuit GND in not connected to chassis?Also, in which scenario, we need to use resistor/capacitor?

author: PGPG
author_rank: Frequent Contributor
timestamp: March 20, 2025, 02:12:28 pm
content: Quote from: Freshman on March 20, 2025, 03:36:24 amWhen you want to have two separate GND nets at PCB for example to ensure that you connect them only where you want to do it than PCB design program normally will not allow you to connect them by track. Connecting them with 0R lets you satisfy program and also control the point the connection is done.For 0R it is not specially important but if you later find that you should use ferryte bead in this place than with the bigger one you can get lower DC resistance for the same 100MHz impedance.Any difference in price between 0402 and 1210 resistor not compensates a costs of ordering second PCB only because to have bigger footprint for ferryte bead.Quote from: golden_labels on March 20, 2025, 05:09:14 amTo not store 5% resistors and 1% resistors since about 5..8 years I am using only 1% resistors. Difference in their cost related to the rest of the whole project is negligible and you save money by having to store smaller assortment, need to order less separate values (in some cases can order bigger quantities).If you use both it can happen that at one PCB you will have the same value 1% and 5% with effect of bigger costs of preparing assembly machine (I believe they estimate it based on how many spools have to be installed) and if you are unlucky it can lead to need to use bigger assembly machine that I believe make assembly costs bigger.

author: PGPG
author_rank: Frequent Contributor
timestamp: March 20, 2025, 02:30:45 pm
content: Quote from: Freshman on March 20, 2025, 01:58:06 pmEMC is so complicated that probably no one can precisely answer.Imagine for example RFID antenna. Its task is to emit magnetic field, but it emits additionally unwanted electric field. This electric field can lead to unwanted common mode currents.If in front of antenna you will have one (interrupted) coil it can take important part of this electric field but if it will be floating it will give you nothing. You need a way to equalize its potential with the local GND so make a connection.Sometimes the connection is needed for some frequencies and for the other is downright harmful. So the constructions like small capacitor to get short at very high frequencies and ferryte bead (in pararell) to have DC connection but not at higher frequencies.It not exactly answers your question but read articles I have linked here:https://forum.kicad.info/t/decoupling-capacitor-via-placement/25132/14https://forum.kicad.info/t/decoupling-capacitor-via-placement/25132/19