Is my touch screen problem common mode noise? - Page 1
Forum section: electronics / projects

author: loki42
author_rank: Frequent Contributor
timestamp: November 07, 2021, 08:29:51 am
content: I have a touch screen module that when connected via the power supply to certain AC adapters (laptop style AC to DC 15V) does not function correctly. The touch point is random. Other AC adapters work fine. If I connect from the ground of the board controlling the touch screen to my scope ground (or anything with protective earth / 3 pin ground) it works correctly with any AC adapter. I have tried filtering the 2.8V supply going to the touch screen in a range of ways, to see if differential noise is the issue. I've tried fancy ultra low noise LDOs combined with RC / LC / pi filters and each of these independently. These didn't change the behaviour. The problematic AC adapters do have more noise on my scope. I am now thinking it's probably common mode noise, as the finger on a capacitive touch screen will be basically earth.Can I make it work with these problematic AC adapters by using a common mode choke? I would prefer a solution I can place on a small PCB in between screen module and main board so that changes to either of the others aren't needed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2021, 12:51:05 pm
content: Yes, that's normal, albeit somewhat sadly so.Adapters are sometimes poorly filtered (or maliciously so, because of profit margins and irresponsible importers..), and capacitive touch works by reading small electric fields. The signals are on the order of a few volts, so noise on a similar level is likely to cause problems. Completely unfiltered adapters can easily have more.There's also touch current, where the open-circuit voltage can be 60 or 120V (basically half the mains input), though the current is quite small (safe to touch) (at least.. with a properly designed and operating supply, it is). This also has an electrotactile effect: if you've ever ran your finger softly across a surface and felt, what seems like a squeaky surface, or almost a vibration -- that's actually the tiny mains current flowing into you. If you notice this effect, it's likely your touch screen won't operate, either. Give it a try!(IIRC, the electrotactile effect doesn't work at high frequencies, so a noisy supply might not have the effect, but still not register touches.)Tim

author: loki42
author_rank: Frequent Contributor
timestamp: November 07, 2021, 11:10:37 pm
content: Is there any recommendations on how to find the characteristics of the common mode noise and select a choke? I'm assuming a higher impedance rating on the choke will lower the cutoff frequency but most of the chokes seem to list good filtering for quite high frequencies and minimal at lower.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 08, 2021, 09:47:09 am
content: Sure, but it's not likely to solve your problem.A better solution is somehow grounding the phone. Perhaps, something like: charging through a USB hub which is grounded to a metal table you're seated at. Here, it's not so much that the table is grounded to some far-off reference (locally, nothing cares if it's tied to a ground rod, or floating in space), just that the phone and user are at a similar potential, and the safest way to do that is by grounding everything.The best solution is simply getting a good power supply.Tim

author: loki42
author_rank: Frequent Contributor
timestamp: November 09, 2021, 02:03:11 am
content: Is there something that will likely solve the issue that doesn't involve changing the power supply? They are provided by end users who will invariably find the cheapest nastiest power supply available. Trying to explain which power supplies to works with will take a ton of time and not be a great customer experience. The common mode chokes I've so far tried haven't helped. The main board has -12, +12 and +5 power and GND.I assume the two pin AC converter return and in turn the screen ground is too far from the same potential as the user / earth as you suggest, however apart from specifying that it connects to 3 pin PE I don't know how to fix the issue.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2021, 07:19:19 am
content: What is this actually?The normal way a product is provided, it's packaged with a recommended supply/charger, there may be a list of recommended/verified alternatives, and the user is on their own if they choose anything else. You're never going to prevent a user from giving themselves a terrible user experience if they choose to.Tim

author: loki42
author_rank: Frequent Contributor
timestamp: November 09, 2021, 07:24:25 am
content: It's a eurorack synth module (for a musical instrument). So it goes into a rack, and the rack has a common power supply that provides -12, +12, +5 and GND over a ribbon IDC cable. The issue it turns out isn't really AC adapter quality, but more that if neutral is bound to earth (PE). If the output ground is isolated from earth then I have problem, and I'm unsure how to address that.The normal way a eurorack module is supplied is to go into the rack, which follows a relatively fixed physical format and a quite flexible electrical specification...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2021, 07:55:01 pm
content: And the rack itself isn't grounded..?Tim

author: loki42
author_rank: Frequent Contributor
timestamp: November 10, 2021, 12:01:01 am
content: If it's powered by a floating AC adapter then no. The AC adapters usually have 2 or 3 pins and connect over a barrel plug.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 10, 2021, 09:40:19 am
content: Not even through cables to other things?Man, audio stuff in general, is so unsafe... and poorly designed. Ground loop doesn't have to be a problem, but no one knows there's an actual solution.Tim

author: loki42
author_rank: Frequent Contributor
timestamp: November 11, 2021, 09:22:36 am
content: Very true. I've fixed the issue though. The touch panel firmware allowed a frequency hopping mode that works well to avoid the noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 11, 2021, 11:27:43 am
content: Oh cool!Tim