A very basic/stupid question.. I'm designing some LVDS multipoint pcbs, and I'm a bit confused about the source termination on the driver end. my colleague said that ideally there should be a 100ohm parallel impedance matching resistor at the driver output, to reduce signal reflection. but I did some research and 99% of the time, people don't add this parallel termination resistor at the source, and only at the receiver end or the bus end (as shown above).
I understand the termination at the receiver end and bus end, but for the driver end parallel termination, is it really necessary?? my understanding is that if the differential impedance of the tracks is 100ohm, then the driver sees a 100ohm output impedance so adding another 100ohm in parallel wouldn't make a difference. plus the additional 100ohm would load the driver more which could reduce the magnitude of the diff signal.
Can anyone help me to understand why people don't add the 100ohm parallel source termination?
NEW UPDATE: Ok I did some more research today.. and got more confused..
First of all, TI has this notes about how to terminate differential signal: https://www.ti.com/lit/an/snla034b/snla034b.pdf?ts=1617704437563&ref_url=https%253A%252F%252Fwww.google.com%252F
It mentioned series termination on both Out+ and Out- for impedance matching.
In the LVDS session, it mentioned double termination, which as my colleague/supervisor said, has parallel termination at both driver output and the receiver.
My only way to make sense of it is that in the TI document, it assumes the driver is a voltage source and in the SiTime document, it assumes the driver a current source..i know this is a stupid explanation..
it looks like most of the time impedance matching at the receiver will do the job, but if the driver has good enough driving power and we want a super clean signal, then we should add impedance matching at the driver too.
I'm not confident about my reasoning, it feels like i'm trying to just fit an explanation to all the things I've read. if anyone can clarify it a bit that'll be great. So my main questions are:
- If we want to do proper impedance matching, should there be source impedance matching as well? and if yes, should it be parallel or series or depends (depends on what?)?
- What is the purpose of the source impedance matching (if needed)? is it to stop signal bouncing back to the driver at the entrance of the tracks, or to prevent second round of bouncing?