Could Someone Review my PCB? - Page 1
Forum section: electronics / beginners

author: ConnorM
author_rank: Contributor
timestamp: December 09, 2017, 03:35:03 am
content: OverviewHi, I am looking for somebody to review my work on the schematic that I've attached. This is on of my first ever attempts at making a PCB/Schematic. This PCB is for a school project, I am in mechanical engineering and I don't know a whole lot about circuitry so I have tried my best.My end goal is to have a PCB with a Teensy 3.6 mounted to it. The Teensy 3.6 will take in filtered analog signals to the ADC's and store the data to an on board SD card at a rate of 10,000 samples/s.The sensors will be:PCB DetailsTwo ADXL377 accelerometers (X, Y, and Z, +/-200g) - One ADXL377 will output X, and Y, the other ADXL377 will output Z (for frequency response reasons), for a total of three acceleration inputsFour FlexiForce A502 piezoelectric sensorsOverall there are 7 analog inputs that must be sampled at 10,000 samples/s.The board layout picture is my first ever attempt at a board layout. Please let me know of any changes that you would make and how I can make it better.The Teensy3.6 Breakout schematic shows my connections for the accelerometers, power, header for testing, and header for communication with another board.The Force Sensor OpAmp schematic shows my non-inverting op-amp setup with the MCP6004 (using this op-amp because the force sensor manufacturer recommends it). I am using the op-amp in single rail setup and I will account for the DC offset in post processing.The op-amps circuit takes 3.3V from the Teensy 3.6 and passes it through a voltage divider to make it a 0.25V signalThe op-amps output changes based on the resistance of the piezoelectric sensor (i.e. any force change on the sensor will cause a resistance change)The output signal will have a dynamic range of 0.25V - 3.3V, this is passed through a voltage divider to only allow a maximum of 1.2V through to the Teensy 3.6 ADC. A maximum of 1.2V is needed as the Teensy 3.6's ADC has a stable 1.2V ref that it uses to convert analog to digitalPlease let me know your thoughts on my work so far as I would like to get these manufactured soon!

author: ataradov
author_rank: Super Contributor
timestamp: December 09, 2017, 04:11:15 am
content: Get a set of DRC rules from any board manufacturer and run a DRC check.Your traces are way too thin, and there is no reason for it on a board that sparse. In some places your traces are way too close to each other and that's also unnecessary.Layout with diagonal lines looks really strange and hard to read. Layout with lines not at 45 degrees makes me twitch.Resistors seem to be way too big. And if you are using SMT capacitors, why not SMT resistors as well?Resistors are placed chaotically. You want to keep your loops tight with opamps.Distribution of connections by layers is also all over the place.

author: ConnorM
author_rank: Contributor
timestamp: December 09, 2017, 04:24:49 am
content: I will work on the component placement.I am using OSH Parks DRC rules.I suppose I shouldn't have used the auto routing tool, or maybe I used it incorrectly...I have to solder the parts and I haven't done much soldering before so I tried to keep everything through hole mount. I could try going for surface mount components, but I'm not sure if my soldering skills are up to it.

author: ataradov
author_rank: Super Contributor
timestamp: December 09, 2017, 04:27:47 am
content: Quote from: ConnorM on December 09, 2017, 04:24:49 amAutorouting is not just pressing a button. It is setting up rules. And a proper setup for a simple board like this will probably take longer than manual routing.Quote from: ConnorM on December 09, 2017, 04:24:49 amThen why not use through hole capacitors as well? Or use bigger size of SMT components.

author: ConnorM
author_rank: Contributor
timestamp: December 09, 2017, 04:32:01 am
content: Now that I think about it I believe that I had these capacitors picked out based on previous design iterations, I'll look for some through hole mountable ones at the req'd capacitance.Do you know of any good tutorials for learning how to route a PCB? Thanks for the help.

author: ataradov
author_rank: Super Contributor
timestamp: December 09, 2017, 04:38:25 am
content: Quote from: ConnorM on December 09, 2017, 04:32:01 amNot really. I learned a long time ago with paper and pencils, so I have not tried to look for tutorials. Someone else may be able to help here.Just do it over and over, I guess. With 2 layer board with analog functions, you want to keep majority of the routing to one layer, and pour the second layer with a ground polygon. Keep things straight and more or less aligned to a grid. And do a DRC check as often as possible at first. It will tell you what's wrong.You can find OSHPark DRC rules herehttp://docs.oshpark.com/design-tools/eagle/design-rules-files/. They are representative of an average prototype manufacturer. But you can tweak them to your taste.And may be look at some other routed boards, and see how they look.

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 09, 2017, 10:14:27 am
content: Also no tutorial here. But probably some hints that works for me:1. Place devices that must be at specific positions. For example connectors. Sometimes it is also better, if possible, to change the polarity of connectors.2. If your board has some function blocks. The parts usually belong together --> Place them together.3. If you have sensitive analogue signals, see if there's some way to part them fromnoise makers(also signal traces could be source of electrical noise).4. In the ratsnet view, after roughly placing the devices to the potential final place: Rotate the devices to see if one orientation might have shorter routing. Most of the time this ends up in replacing several parts (e.g. if a bus is now up side down).5. After routing some traces do a sanity check. Sometimes one over-/underestimate routing areas. This sometimes cause to go back to point 2.6. After finishing the routing usually fill the areas with GND. But make sure if you have separated the analogue and digital GND. Usually the tool don't know that you want to have them split.In the beginning you probably have some more iterations. Think about it like a puzzle.Wrt the SMDs parts: If you have some experience in soldering you should think if you can go SMD. Especially if you're using the big SMD parts. Some libraries have special parts marked for hand-soldering. These footprints have overly large pads.If you have some time you might check if there is some cheap kit which have some SMD parts to solder. So you can try if you're capable to do this. But Resistors, Caps bigger diodes ain't that hard to solder.

author: jgalak
author_rank: Regular Contributor
timestamp: December 09, 2017, 03:21:23 pm
content: I'm not sure what software you are using, it looks like Eagle to me. If so, Sparkfun has some good tutorials. Go here :https://learn.sparkfun.com/tutorials/designing-pcbs-advanced-smdand then scroll down just a bit to "suggested reading". That lists all the eagle tutorials.