My 2nd attempt on a hall-sensor based fluxmeter  - Page 1
Forum section: electronics / projects

author: johofz
author_rank: Contributor
timestamp: December 17, 2020, 11:49:48 am
content: Hello dear eevblog community,I'm working on a project for a small company I recently started at. Since I'm the only Engineer with some ee-knowlage (studied automotive-engineering), I'm in charge of building all the electronics for test benches and such. The project I'd like to show here is a hall-sensor based fluxmeter.It's already version 3. The first version was a proof of concept using a stm32-nucleo board and building ontop of that. The second version was a more integrated solution with a STM32F405 Chip on a single-board setup. The new version I'm going to show here is supposed to improve the second version and add some more functionallity.The general setup is as follows:- 5x hall sensors measure the field across some magnets (exact application is irrelevant)- 1x temperatur sensor for temperature correctionThe sensors are connected via a VGA-cable (since we have a lot of them around and they have more or less the amount of shielded cables we need)to the PCB. The whole thing is USB-powerd and sends data via USB to a Host PC where the data gets processed.The analog side consists of:- 1x 8-Channel 12bit ADC (ADC108S022CIMT/NOPB -https://tinyurl.com/yctd2x7p)- 3x 4-Channel rail-to-rail Opamps (MCP6004T-I/SL -https://tinyurl.com/ybdgdrpf)- 2x 3.3V Reference - for ADC and sensors (REF196GSZ-REEL -https://tinyurl.com/y9y6aqlf)I use simple amplification circitry with adjustible offsets. Since the signal from the hall-sensors will be sinusoidal and I'm intrested in phase shift and frequency, I included two zero-crossing-detections in hardware with non inverting schmitt triggers (also adjustable offset).The ADC output gets processed from a STM32F405RGT6 (https://tinyurl.com/y8hytc2j) and either send via USB to a host PC or displayed on a 1.8" TFT display (or both depending on the performance impact).Since this was my first time implementing a STM32-Chip (in Version 2) I was really happy with how it turned out. I did mess up some things in the design (mostly dumb mistakes like choosing non rail-to-rail opamps by accident) and I wanted to have the option of displaying some stuff via a display. Thats why I redid some things. I also did not check the VGA pinout and connected 3.3V to the VGA shild. Luckiely it did not do any harm. I still want to correct that ofc.What I'm mostly interested in, is the grounding of the chassie and if it's done correctly. Also I'd like to know if I did some other major mistakes.It's a 4 Layer PCB with Signals on top and bottom, separat ground and 3.3v planes in the middle. I did my best to implement all the best practices I know of. But I'm still a beginner in therms of PCB-Layout. I have a deeper knowlage on the software side.Feel free to tell me what I could do better! I'd love to here some feedback!Greetings from Germanyjohofz

author: ElectronRob
author_rank: Contributor
timestamp: December 17, 2020, 06:02:51 pm
content: Looks methodical and neat. You software types don't like decoupling caps much though do youAdd decoupling caps to the opamps at the very least, but shoot for adding them to all components. Go wild and use 100n-1u.The vias going from top to bottom near the bottom left of the board (SPI3), the way they extend beyond the track is considered to be poor practice by many, however its unlikely to cause you any problems.Vias too close to pads, close enough that the solder mask doesn't exist between pad and via is also bad practice and many of your resistors/caps seem to be awfully close (without need).No harm in flooding top and bottom empty space with GND.What are you hoping to use the shield for? best I can see from schematics you just have earth and GND connected via a single 220p cap or a solid jumper. To be effictive for EMC your chassis should be connected to GND via a pretty low impedance connection, while respecting potential ESD. Here I would add a parallel (with your 220p cap) resistor, in the order of 10-100R, change depending upon results.In general I do not make a ring connection as you have, loops are bad, break it.Your 220p cap is small (size wise) any sizable esd even will jump it. Usually these are 1206 min with a high voltage rating. However I suspect you are trying to use the chassis ground to shield from noise rather than to pass emc. In which case just break the loop and ground it, junk the cap. I would normally connect chassis to GND near to I/O cabling (noisy cables which enter your box).

author: johofz
author_rank: Contributor
timestamp: December 18, 2020, 08:07:02 am
content: Thanks for the input! Much appreciatedQuote from: ElectronRob on December 17, 2020, 06:02:51 pmThankfully I don't intend to leave the linear region of the opamps (except for the schmitt-trigger configuration ofc). I will keep that in mind for futur designs!Quote from: ElectronRob on December 17, 2020, 06:02:51 pmMy intension with the shielding was mainly for it to shield against EMC and noise since we have a lot of noise sources in our "lab". I mainly read this posthttps://www.eevblog.com/forum/projects/plated-mounting-holes-grounded-or-not/and this onehttps://www.eevblog.com/forum/beginners/signal-ground-vs-chassis-ground-should-they-be-coupled/.What I took from that was a lot of confusion and basicly what I came up with in my design. I choose the jumper instead of a 0 ohm resistor to be able to test with and without the connection. I can always solder a THT 1M resistor through there if need be. That was the thought process at leastI will break the loop like you suggested, since its also suggested in the posts i mentioned to only have a single connection to avoid loops. I thought the Jumper and the cap close to each other would do the trick. The chassi is all metal. I think I didn't mention that. ESD could be an issue, so maybe i will end up with a 1M THT resisitor instead of the jumper.