I2S digital expansion port, wondering about EMI - Page 1
Forum section: electronics / projects

author: jrs45
author_rank: Regular Contributor
timestamp: July 09, 2021, 09:59:10 pm
content: I have a audio power amplifier designed, and I'm considering adding an I2S port (FFC) to drive a second board. These signals are ~25MHz (but may have a sharp risetime). 3.3VThe most convenient connector position is at the end of ~2" traces that carry all these signals. But wouldn't these high speed, unterminated traces, all turn into antennas transmitting all kinds of noise? Or is it very low because the current is effectively zero? (I'm not sure what the RF impedance would be of each trace over a ground plane in a 4layer PCB, maybe I should figure that out.)edit: Since it's on the outer layer of a 4 layer pcb, 8mil tracks with 9mil thick dielectric, it's 74 ohms stripline for each signal line.

author: Benta
author_rank: Super Contributor
timestamp: July 09, 2021, 10:27:55 pm
content: The standard way of doing this on a 667 MHz DDR bus is series termination at the receiving end (18 ohms) to dampen ringing. At 25 MHz, I wouldn't worry too much.

author: Bassman59
author_rank: Super Contributor
timestamp: July 10, 2021, 05:21:58 am
content: Quote from: jrs45 on July 09, 2021, 09:59:10 pmI did something similar ages ago, forwarding I2S plus the modulator clock between boards. I used a quad LVDS driver DS90LV048AA and its complementary receiver DS90LV047A. I used standard RJ45 jacks (not Ethernet MagJacks) and the connector pinout was the same as Ethernet so off-the-shelf CAT5 patch cords could be used. To help meet EMC I put ferrite beads on the output signals and we had clamp diodes on the inputs. MCLK was the fastest signal at 24.576 MHz. It worked well.

author: jrs45
author_rank: Regular Contributor
timestamp: July 10, 2021, 05:34:24 pm
content: Thanks for such a thorough response!Quote from: evb149 on July 09, 2021, 10:36:23 pmCorrect, I have inserted series resistors (25 ohm) at the source for this purpose, so perhaps that will be sufficient. From my probing the corners look reasonably round.QuoteYes, these are running along the top and bottom sides, with unbroken ground areas directly under them. Vias have ground vias next to them too. I've also kept any sensitive analog traces well away from this. The parallel traces are all clock, I2S, I2C. I think I have followed good layout practice, it was really the inclusion of these "long" traces that (usually) don't go anywhere but an empty connector that I'm concerned about.QuoteYes, I've included GND on every other pin of the FFC connector for this reason.QuoteI've done all of this! And I've read all of those sources, and they're excellent. I'm also watching everything I can find from Bogatin and Hartley. I think I've got it all correct, it's just the matter of leaving these "long" energized traces that are unterminated that I was worried about. Perhaps that worry is not well justified as long as the rest of the layout is done properly.I just didn't want these things to become antennas.

author: jrs45
author_rank: Regular Contributor
timestamp: July 10, 2021, 05:37:48 pm
content: Nice. This would be for another small power amp nearby (~6") so I don't think I need to get this complicated. Did you place the ferrite beads in series with the signals? Is clamp diodes the right solution to prevent ringing? (Seems hamfisted but maybe not?)Quote from: Bassman59 on July 10, 2021, 05:21:58 am

author: jrs45
author_rank: Regular Contributor
timestamp: July 10, 2021, 08:41:45 pm
content: Thanks! There will be no cable connected normally, and I can't shut these off (they drive another amp channel normally, this is for an extra identical channel receiving I2S in parallel). It's just the trace stubs I'm worried about, not a dangling cable.