How to overcome EMI in 3.3v logic - Page 1
Forum section: electronics / projects

author: tomshirvo
author_rank: Regular Contributor
timestamp: December 09, 2015, 04:28:04 am
content: Hi All,So I have built a device that controls a system that includes 3.3v logic levels and high current 24v to 72v devices.My controller is in a box and a 2 meter cable with 44 3.3v IO is connected to the main device. I also have a 17 pin cable that takes all the high current loads (72v 4A stepper motor, 24v 5A fans, 11* 24v 0.4A electromagnets).I am having huge problems with EMI stuffing with my logic getting back to my ATSAM3X8E.I know things like shielding and such will help with this problem but I have done as much as I possible can with this current design. I am working on a new PCB and working towards keeping all of the high current/high voltage away from the low voltage IO (I thought what I had done was enough, I was so wrong).I was thinking that maybe adding the ability to run my IO at 24v over the 2 metre cable and then converting it back down to 3.3v if needed at the other end.Would just using NPN transistors work fine for doing this or is there something else out there that would work better or should I just work on keeping everything isolated between high and low voltage?Thanks in advance!!!!!!

author: marshallh
author_rank: Unknown
timestamp: December 09, 2015, 04:31:44 am
content: You really ought to be using differential - whether it's rs485, CAN bus, etc.Any single-ended wire is going to be susceptible at 2 meters long.

author: tomshirvo
author_rank: Regular Contributor
timestamp: December 09, 2015, 04:37:35 am
content: Hi marshallh,These IO are connected to things like inductive proximity sensors and such that float for off and pull to GND when on.Does this mean I should think about putting another controller in the device and just communicate to it over RS485/CAN then I don't need to worry about all the other long cable lengths.Thanks

author: Paul Moir
author_rank: Frequent Contributor
timestamp: December 09, 2015, 04:58:26 am
content: If you can send enough current down the lines to the sensors you may be able to overcome the noise. The old industrial method was to use 4ma-20ma current loops, 4mA being "zero" and 20mA being "100 %". The 4mA thing was so you could detect an open sensor.Can your sensors take 10s of mA?Even then, the sensors usually got their own shielded cable.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: December 09, 2015, 05:48:57 am
content: What is the nature of your EMI issue?For instance:1. Malfunctions in your lab during operation.2. High radiated emissions at test lab.3. High conducted emissions at test lab.etc.If you haven't been to a test lab yet, it may be worth while to get test time at one to determine how much over the test limits you are. The data won't be applicable to any finalized testing for reports you might do, but it can reveal a lot.

author: Kalvin
author_rank: Super Contributor
timestamp: December 09, 2015, 07:50:58 am
content: Using opto-couplers may also help, as the voltage controlled logic is turned into current controlled logic. The opto-couplers should be placed to the driving PCB ie. the same PCB where the controlling MCU resides in order to reduce EMI problems.As others have already suggested, differential signaling and current loops can also be used to solve the EMI problems.Proper PCB layout and cabling is necessary no matter technique you end up using.

author: Fungus
author_rank: Super Contributor
timestamp: December 09, 2015, 03:34:37 pm
content: Quote from: tomshirvo on December 09, 2015, 04:37:35 amWhat value pullup resistors are you using? Try using smaller values.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 11, 2015, 02:57:17 am
content: Controlling the dV/dt and dI/dt of those power signals will go great lengths, if that's under your design control.Keeping logic signals close to ground does wonders, but won't prevent errors (the signals are still fundamentally unbalanced, and therefore radiating some signal, and being susceptible to outside signals).Layout and design examples:- 4 layer PCB with internal ground plane (traces on the outer layers run close to ground, also having reasonably controlled impedances).- 2 layer PCB with low density layout (wherever possible, overlapping of traces is avoided), so that ground pour can fill the top and bottom. Plus liberal stitching to make the two pours act as one solid ground plane.- Ribbon or multiconductor cables where every other wire is ground; twisted pairs where one is ground; etc.- Filtering signals wherever possible, especially for nets leaving the PCB (i.e., on cables, even short ones). Use only the bandwidth you need!- Use signaling standards suitable for long distances, high immunity, etc.Differential helps, but isn't a panacea because few standards are truly differential (uncaring about common mode voltages).Example: LVDS, a ~100mV signal. Pretty vulnerable right? But it's differential, yay! ...But how differential? Well, it can't go outside the ~3.3V supply range of whatever the receiver is operating on, or it just goes into cutoff and reads dick. If the bias is 1.65V (half supply), you get +/-1.65V of range. Not very much. Certainly more than it started with (e.g., the competing signaling standard in computer hardware is SSTL and the like, which have a 100mV thresholdwith respect toGNDVREF, i.e., single-ended, so a trace gaining only 100mV of interference is screwed), but nothing you'd want to put into a cable (where the IEC standard, and FCC recommendation, is about 3V worth of interference -- deal with it as best you can).The most popular bog-standard signals, like RS-232 and -422/485, work by being larger (or beefier) and slower than most noise signals, and having enough working range to deal with it when it does show up. RS-232 is a whopping 15V (10 minimum), bipolar, so it's good enough by sheer amplitude.RS-422/485 is lower voltage (3V difference, at most), but terminated, so the current flow is quite large (compare with the physics of 4-20mA current loops), and the common mode receiver range is +/-7V.Plus both are usually used quite slowly (say, <= 115.2kbps), so substantial filtering can be added, preventing almost all interference (they only test at >= 150kHz for conducted susceptibility).Among popular, fast, modern standards, USB is awful (just like LVDS, it's limited by a 3V receiver supply range), and requires enough bandwidth that filtering is ineffective (Full Speed uses about 20MHz total bandwidth). This is why USB always requires shielded cables! Whereas Ethernet is superb (it's freakingtransformer coupled, as differential as you can get).Tim