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HasanTheSyrian_ , 09-02-2025, 03:28 PM

Youre supposed to put series termination as close as possible to the driver however in my board it is not possible.

I pretty much have have them right next to the receiver. Will this create any problems for signal integrity, reflections etc...

QDrives , 09-02-2025, 08:23 PM

It is the blue versus the red trace.

Karthik , 09-03-2025, 07:44 AM

It might be better to not have the series termination in this case compared to having it near the receiver. In your current scenario you have some (20R-30R?) driver impedance, then a 50R transmission line, then a series resistor (22 - 33R?) and then the receiver impedance(50R or high impedance?) . You might be better of having just one mismatch at the driver side. Try and run a sim and compare...

HasanTheSyrian_ , 09-03-2025, 09:27 AM

The problem is that I have parallel signals going to the FPGA SOM which has differential pairs meaning the 100 ohm pairs will be used as 50 ohm single ended traces that are 8mil to 0,2mm apart

HasanTheSyrian_ , 09-03-2025, 09:28 AM

and the trace length is about 120mm so im concerned about the crosstalk and reflections - the reference designs and datasheet say to use termination resistors

HasanTheSyrian_ , 09-03-2025, 09:28 AM

if you are suggesting that using no resistors is better, can i keep the resistors and if there are issues i can just place 0R resistors instead?

HasanTheSyrian_ , 09-03-2025, 09:29 AM

is the red trace the blue signal with termination next to the receiver?

Karthik , 09-03-2025, 10:11 AM

Based on what you say, the receiver impedance is 50R single ended and not high impedance, so may be having no series resistor is better (leave footprint and fit 0R) than having one near the receiver. But please dont blindly take my advice, run a simulation if you can to confirm. What is the frequency of these signals?

HasanTheSyrian_ , 09-03-2025, 10:34 AM

~150 mhz 16 parallel lines

HasanTheSyrian_ , 09-03-2025, 10:34 AM

as far as I know rise time matters more than frequency

QDrives , 09-03-2025, 01:53 PM

Blue trace is with the series termination at the driver. The red trace is the series termination at the receiver.

QDrives , 09-03-2025, 01:55 PM

I assume those are **parallel terminations** and not series? Big difference!
Series termination is at the driver. Parallel termination usually at the receiver.

HasanTheSyrian_ , 09-03-2025, 01:56 PM

the signal consists of parallel single ended lines

HasanTheSyrian_ , 09-03-2025, 01:57 PM

the termination resistors are in series on each line

QDrives , 09-03-2025, 03:30 PM

What is your rise time and trace length?
You could also try an AC termination.

HasanTheSyrian_ , 09-03-2025, 07:40 PM

1-2 ns about 150mm

QDrives , 09-03-2025, 07:43 PM

Well, that would require termination.

Karthik , 09-04-2025, 07:22 AM

Depends how you want to analyse it. I like looking at signals in frequency domain and typically simulate S-parameters upto 7th or 9th harmonic of the fundamental to see how the PCB interconnect performs.

HasanTheSyrian_ , 09-04-2025, 07:51 AM

do i use parallel termination instead then? pull the resistors down

HasanTheSyrian_ , 09-04-2025, 08:01 AM

i really have no choice with the resistors placement lol you can see the SOM connector is right there are the bottom

Karthik , 09-04-2025, 08:02 AM

Can you rotate the connector 180deg?

HasanTheSyrian_ , 09-04-2025, 08:02 AM

no

HasanTheSyrian_ , 09-04-2025, 08:06 AM

the dev board that my SOM uses has an HDMI output like mine but they use a different IC, SIL9022 instead of ADV7511. That IC takes 16 parallel inputs like mine and the frequency is about the same. They have connected the single ended inputs to an LCD connector and placed termination resistors there

HasanTheSyrian_ , 09-04-2025, 08:06 AM

obviously the driver in this case is the same (same som same fpga same rise time)

Karthik , 09-04-2025, 08:08 AM

As you may know, series termination aims at making the driver impedance 50R but parallel termination is aims at making the input impedance 50R. From what you say, your input is already 50R, so not sure what parallel resistor will achieve..

Karthik , 09-04-2025, 08:09 AM

Its hard to say, best method is to run simulations and see... sorry not being very helpful here !

HasanTheSyrian_ , 09-04-2025, 08:10 AM

the input out impedances are not known they are almost never mentioned in datasheets as you know, im following reference designs and what the datasheet says

HasanTheSyrian_ , 09-04-2025, 08:10 AM

the problem is that in my case there is a som

Karthik , 09-04-2025, 08:16 AM

If its CMOS driver, its roughly 25R -30R and hence the recommendation of a series resistor. In your case, if you cannot place the series resistor near the driver, then may be it serves no purpose as the impedance mismatch has already happened when the impedance controlled PCB trace starts from the output driver. Adding a 22R resistor elsewhere on the transmission line only makes it worse imo.

HasanTheSyrian_ , 09-04-2025, 08:35 AM

at the trace length of 150mm dont i need termination though

HasanTheSyrian_ , 09-04-2025, 08:55 AM

i can decrease the drive strength too i guess

HasanTheSyrian_ , 09-04-2025, 10:23 AM

this is a simulation of the rising and falling edge using the ibis models of the fpga block design exported and the receiver with no termination, 3.3v, 8mA drive strength, 120mm, SOM stackup and trace widths

HasanTheSyrian_ , 09-04-2025, 10:26 AM

this is 12mA drive strength

QDrives , 09-04-2025, 10:32 PM

Do note that the connector is not the beginning / end of the signal.
So are the yellow pins on the IC receivers or drivers?
For receivers, you should assume that the driver already has series termination and that is before the contra connector.

HasanTheSyrian_ , 09-05-2025, 07:03 AM

the ics you see are the receivers

HasanTheSyrian_ , 09-05-2025, 07:03 AM

i think my only option is to control the drive strength and have no termination

HasanTheSyrian_ , 09-05-2025, 07:03 AM

ill put 0R resistors just in case

Karthik , 09-05-2025, 08:36 AM

Given your situation, i dont see many choices. Having a series resistor placeholder is good for peace of mind 🙂

HasanTheSyrian_ , 09-05-2025, 09:54 AM

what about a signal repeater

HasanTheSyrian_ , 09-05-2025, 09:54 AM

some active driver

Karthik , 09-05-2025, 10:25 AM

Do you have space for that? And secondly you still ideally want to terminate the interface to that driver, no?

HasanTheSyrian_ , 09-05-2025, 10:54 AM

no lol

HasanTheSyrian_ , 09-05-2025, 10:54 AM

what odds do you give this actually working

Karthik , 09-05-2025, 11:16 AM

Gut feel says it will work...but then it also depends on what connects to the other side of the connector -how long further the signals have to travel before reaching the driver / receiver

QDrives , 09-05-2025, 03:12 PM

Then again, the connector is **not** the driver. The driver is on another board that also has a connector.