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Mahsheed , 05-21-2026, 06:08 PM

I am using 120 ohm termination resistor in 422 differential pair routing. Since my resistor is of bigger size, so it disrupts the differential pair as it could be seen in the image. I want to know if it is standard/good practice to add termination resistors in differential pairs like that?

, 05-21-2026, 06:13 PM

🤖 In the meantime, here’s a response from the [FEDEVEL AI HW Assistant]().
It helps to find relevant resources for your question, but it may make mistakes as it’s still in early development.

This is the response from the AI Assistant:
For RS-422 differential pair termination, here's what matters based on the video content:

1. **Termination Placement**: The 120Ω resistor should be placed at the **far end** of the differential pair (receiver side), not inline. This matches the differential impedance and absorbs reflections. [Source](https://www.youtube.com/watch?v=h9yyOmL8duE&t=751)

2. **Physical Implementation**:
- Use **smaller packages** (0402/0603) whenever possible to minimize trace separation
- Maintain **constant differential impedance** through the resistor by:
- Keeping traces symmetrical
- Using tight return path reference planes
- Avoiding abrupt width/spacing changes

3. **If Using Larger Resistors**:
- Route traces **under the resistor body** if package allows
- Keep P/N traces **parallel and equidistant** through the component
- Avoid creating stub traces or impedance discontinuities

4. **Validation**:
- Check TDR response for impedance mismatches
- Verify common-mode rejection ratio (CMRR) in final implementation
- Consider EM simulation if signal integrity is critical [Source](https://www.youtube.com/watch?v=nNvBNXQ7zKI&t=768)

The videos show that while compact routing is ideal, properly implemented termination with larger components can still work if impedance control and symmetry are maintained. Simulation examples demonstrate that even with physical disruptions, careful layout preserves signal integrity.

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Mahsheed , 05-21-2026, 07:08 PM

or is this a better way of doing it or anyother way to do it?

Robert Feranec , 05-23-2026, 07:30 AM

what are the dimensions of the 120OHM track? how did you calculate it? it looks very thin

Robert Feranec , 05-23-2026, 07:34 AM

i would not route tracks between pads ... if for anything else that for the reason that the pad and track are the same net and some CAD softwares will allow you to route them closer than what is minimum allowed clearance (because they are the same net some CAD sofwares will not consider it as a violation - this means that if they are routed closer than minimum clearance there may by problems during PCB manufacturing)

Mahsheed , 05-23-2026, 08:02 AM

Thankyou very much for the detailed reply
Please look at the attached picture of JLCPCB's impedance calculator. The track width is 0.129mm
track to track min clearance is 0.127mm and track to pad is 0.254mm. And yes, the pad and track are of same nets. Basically these are the pads of termination resistors in 422 differential pairs.

If I do not pass the differential pair like that, then how should it be done? this way, I have no disturbance in differential pair routing and minimum stubs that connect resistors to that differential pairs.

Termination resistors are actually placed between + and - tracks at the receiving end. so what could be the better way of placing such resistors?

Robert Feranec , 05-25-2026, 05:25 AM

i would not worry about 422 so much. Normally I route from pads to pads, I try do not route between pads

Robert Feranec , 05-25-2026, 05:25 AM

the main reason is, that I would like to keep the clearance between the pads as big as possible

Robert Feranec , 05-25-2026, 05:28 AM

especially i dont want to risk that for some reason the tracks under the component will not have proper mask and it could cause short circuit. I am not saying that it is a big risk, it is just my habit