measurements first, smd rework second.
BTW… oh shi~
I looked into a schematic of USB Client DP (version unspecified).
There are almost none of those ubiqitious .1uF ceramic capactiors. A good engineering rule of thumb says “bypass every noise-producing or noise-sensitive device pin with a ceramic capacitor close to the device”.
But no! there are only two of them.
(a) C1, almost well-placed bypassing the output of IC9 (MIC4680 buck-mode 5 V regulator), however the ground trace is too long and too curved and may prevent it from doing its job properly.
(b) C3, could be reasonably well placed near IC1 IN1 (pin 8). However, it’s futile there: IC1, a TPS2115 power multiplexor* doesn’t seem to produce a lot of RF and spikes most of the time.
The proper places for the ceramic capacitors are, with every cap providing an RF bypass path to the ground at a power line it’s connected to:
(1) near the barrel connector before any diodes
(2) near the mini-USB connector
(3) at the output of IC9 critical (MIC4680, the 5V switching regulator, mistakenly labelled -ADJ in the sch)
(4) at input to IC9 important
(5) at the output from IC7 (LM1117, 3V3 linear regulator) required by manufacturer
(6)(7) at both inputs to IC1 (TPS2115, power mux) required by mfg
(8) at input to IC7
(9)(10) at the 2x5 connector.
Caps with notes in bold face MUST be on the board no matter what, caps without bold face SHOULD be there for us to live in a better world.
All caps should be placed near the associated pins. Copper artwork should feature good RF-quality polygons (both live and ground): short and wide, without sharp angles and excess bends.
I dunno why everyone skimps on the ceramics, they are really really cheap and the board’s aren’t quite a bargain.
- Input power mux is a good thing to have, but it doesn’t work on me: when transitioning from external to USB power, the device crashes the moment I unplug the barrel.
Note: at least there are no reports on the regulators blowing up at random or after being touched
(like it was the case when Atmel screwed up with the AVR Dragon in-circuit debugger/programmer for their 8-bit lineup)
Note: I’m not talking about polar capacitors (aluminium and tantallum) here, they are an entirely different matter and serve a different purpose. And they do almost nothing when it comes to fighting noise.
Side note: data lines on the USB Client DP are good, nothing is left floating and generally doesn’t require bypassing.
I will provide additional “end user” advice on aftermarket patch-up somewhat later.