Series termination resistors and impedance continuity required for APA102 pixels?

A

Apa102

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I know that it’s good practice to put a 100 ohm resistor before the first pixel’s data and clock line. However I’m using individual dotstar’s/pixels and not pixels already integrrated into a strip.

The setup is intended to be a single pixel on a pcb which then has each data and clock travelling across a twisted pair cable each paired with ground for about 6 inches then connected to another single pixel on a pcb. Since there is a change on which the signals are travelling on (pcb trace vs cable) there will be an impedance mismatch.

The pixels will be spaced about 6 inches from one another. Currently designing a pcb on which the pixels will be mounted on and since the pixel’s are being spaced apart and not as close together as they normally would be in a strip, the data and clock line between them would effectively become transmission lines.

So, would that necessitate me putting a series termination resistor on the output of each pixel’s data and clock lines if they are being spaced in this fashion?
 
PaulStoffregen said:
The resistor is unlikely to be needed for only 6 inches.
Click to expand...

If I intend to have several (up to forty) pixels arranged in this way, there would be about 20 feet in distance from the first to the 40th pixel. Would using resistors at that distance become necessary then?

My understanding is that the signal is regenerated at each pixel so would adding a resistor with a value that’s been properly matched to the impedance of the transmission lines added to the output of each driver/ic output on each pcb as a precaution be useless/mildly helpful or harmful to the signal lines?
 
Thanks for the article and while it cleared up some things, I don’t think it answered my initial question.

Looking at this link: https://www.pjrc.com/store/octo28_adaptor.html
“Together with the OctoWS2811 adaptor's 5V buffer chip and accurate 100 ohm impedance matching to quality CAT6 cable, this wiring style gives a LED project the best quality signal and power delivery.”

As I said in my first message, it seems that it’s necessary to include resistors that much the impedance of the Cat6 cables. Assuming that the resistor value of 100ohms was chosen to match the characteristic impedance of the Cat 6 cables which have (based on my quick google search) a characteristic impedance between 85-116 ohms.

From my understanding, mitigating reflections can be achieved with maintaining the characteristic impedance, which totally makes sense at the beginning of the strips which are usually fed the data and clock signals from a microcontroller which might be several feet away from the first pixel. But wouldn’t it also be necessary at any point that the data and clock signals travel on distances that become transmission lines?

You mentioned that a resistor is unlikely for 6 inches. But for example, what if a strip which used one data and one clock line each, was then cut in half and spaced apart a couple feet, and the data and clock lines of the first strip travelled for 3 feet before being connected to the data and clock lines at the beginning of the first strip. Would one be necessary then?
 
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