USB DAC PCB layout - interference noise problem - Page 1
Forum section: electronics / beginners

author: FartingNoveltyToyEngineer
author_rank: Newbie
timestamp: February 05, 2025, 02:01:44 am
content: I Have designed this USB dac & Headphone amp which has a severe noise issue from cellular network activity. It works okayish with a PC using long cable, but it's completely unusable with a smartphone, especially when placed in a same pocket.Now i'm working with a second revision, which has following improvements:-Changed from 2 to 4 layers (ground on both inner layers).-Placed the components so that no ground currents are flowing through the dac ic area.-Added pi filter to the USB vbus.-Headphone jack ground connects to plane near vbus bypass caps.-Added a guard trace around the analog signals (might have misunderstood that concept)I would greatly appreciate any advice or theories as to what might cause this issue. Is my new revision any better? Should i use ferrite beads somewhere?

author: shabaz
author_rank: Frequent Contributor
timestamp: February 05, 2025, 05:38:31 am
content: Hi,It's not easy to review the PCB layout with just those images, but it will be worthwhile to pause, step back and reconsider how you're approaching all of this. If it were me, I would not be happy with this design at all. Given the stage you're at, you're way too early to be designing a PCB unless it's an experimental PCB, in which case it needs to be larger with space for all your tests/experiments.Some points in no particular order:1. It should of course be 4-layer, unless there's some mass production cost-optimization reason that justifies anything else2. Put three LDO's for the three supply voltages to the DAC, so you can see the difference, and can patch one or two or all three, so you can experiment with the combinations and see what works3. You need pads in series for those supply connections to the DAC, so you can either short them, or insert a ferrite or a low-value resistor, again to see what works for you4. You've not followed the datasheet recommendation for the audio amp, which mentions 10uF capacitance; you've got 2,2uF.5. There's a good chance you will need a regulated supply to the audio amp, to act as a power supply filter; your experiment board therefore should contain (say) an LDO for 4.5V or in that ballpark, which again you can patch in and test with.6. You ought to have space in line with the audio amp supply rail to allow adding a ferrite bead if needed7. Consider using a more modern audio amp, e.g. TPA6132A2 which has much better PSRR8. Check the datasheets, eval board documentation and Gerber files for each component, to see if there are recommended PCB layouts or notes9. Add a signal from that SCK pin on the DAC to your I2S bridge (through a zero-ohm resistor so you can experiment with removing it) if your I2S bridge can supply a clock.10. Have a small resistance (can always be replaced with a zero-ohm resistor if you find you don't need it) in series with the I2S signals between your bridge device and the DAC11. Is there any ESD protection on the USB end? I can't tell, but if there isn't, then consider adding that.There are probably many other issues, but the above is a start.

author: radiolistener
author_rank: Super Contributor
timestamp: February 05, 2025, 06:46:43 am
content: I think using 5V directly from USB to power audio amplifier is a bad idea. Smartphone almost never had stable 5V and its voltage can significantly drop during transmission and it can lead to noise on the output. So, its better to add some LDO and using for example 3.3V to power audio amplifier. Add ferrite beads and low pass filters before LDO to cut-off high frequencies. Use separate LDO to power digital and analog circuits.Also try to experiment with adding resistors and blocking capacitors to ground for audio signals to avoid high impedance lines, high impedance line is very sensitive for EMI interference.And since it will work close to smartphone which can emit up to 2-3 W UHF power, there is a sense to put your circuit into shielded box, otherwise you will have some interference even with good circuit and PCB design. For example, I can see pulse noises at about -90 dBFS around DC on low impedance ADC input from WiFi router working at 1-2 meters distance when ADC board with no shielding.

author: tszaboo
author_rank: Super Contributor
timestamp: February 05, 2025, 04:09:06 pm
content: Quote from: radiolistener on February 05, 2025, 06:46:43 amYes, USB is not 5V it's 4.5 to 5.whatever with a ton of noise.