Sources of 30kHz noise? - Page 1
Forum section: electronics / beginners

author: Maxlor
author_rank: Frequent Contributor
timestamp: September 11, 2015, 12:54:26 am
content: I've been investigating capacitive touch sensing recently, and it turned out to be a bit more of a rabbit hole than I had thought. Microchip alone is using 3 different methods in their various chips:- CVD (Capacitive Voltage Divider), which works on any chip equipped with an ADC by measuring the final voltage resulting from connecting the charged sample&hold cap with the discharged cap to be measured- CTMU (Charge Time Measurement Unit), which connects a constant current source to the discharged cap to be measured and then determines the time required to charge it to a certain level, and- CPS (Capacitive Sensing Module... yeah. They're not great at abbreviations I guess) which also uses a constant current source/sink, but measures the number of charge/discharge cycles in a certain interval.Maybe this picture helps make it clear what the hardware is doing:Now to my actual question: I'm currently using a touch sensor in CPS mode. According to some application note that I can't find right now, it's vulnerable to noise near the charge/discharge frequency. After some experimentation I've found that my sense cap is being charged/discharged at around 30kHz. What noise sources are there in that frequency region that might influence the CPS result, if I'm evaluating the charge/discharge count at about 8Hz? One thing I can think of is switch mode power supply, which end up having a switching frequency in that region. Other than that, I can't think of much that would be happening in that frequency region. I'm sure there's plenty though, so enlighten me

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2015, 01:32:34 am
content: As far as EMC goes, count onanythingcapacitive touch being completely knocked out by any kind of interference, in-band or otherwise.Therefore, it is critical to device operation that it be a noncritical system (e.g., the touch is a supplement for buttons or other controls) and that its malfunction is immediately recoverable (usually the case, as long as some ESD precautions are followed).FYI, if you ever happen to go to a testing lab, bring a wired mouse for your laptop, you won't regret it...Tim

author: Chris C
author_rank: Frequent Contributor
timestamp: September 11, 2015, 02:10:02 am
content: Fluorescent lights with electronic ballasts immediately come to mind as a possibility. Googling "electronic ballast frequency", the first result suggests a typical range of 20-60Khz. That makes sense, as below 20Khz could produce audible noise. But one still wants to keep the frequency relatively low, because going too high introduces the possibility of interference with radio receivers, and the large glass area of some fluorescent tubes would probably make a good antenna that would be hard to shield. Switching power supplies are easier to shield, and usually run no lower than 50Khz for efficiency reasons.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2015, 03:35:01 am
content: I should add; interference on the level of basic CE (IEC 61000-4-6 and -8) tests: 3V into cables or 3V/m at RF.This is an important figure, because if the equipment is energized by this amount of signal, and the finger touching the pad is not (or is randomly energized, as would be the case for a person standing by the equipment in an RF field), then that voltage (more or less) appears between the finger (acts like ground) and the capacitive sense electrodes. And that signal level is greater than most touch systems operate at, and also enough to trip logic thresholds (i.e., reading digital noise on the input pin, whether energized for sensing or not).Contrary to popular belief, ferrite beads aren't used for filtering -- the impedance is too low, and the impedance they're working against (as an impedance divider) is too high. Rather, they serve to dampen resonances, because that 3V interference test becomes magnified to 10V or 30V or more, at certain resonant frequencies, making things even worse for your product. If you can handle the 3V figure at any other frequency, you can use FBs to stabilize this for all frequencies (sort of). Of course, hardening a capacitive touch system for more than maybe 1V would be a challenge already, so that's your first problem.In most environments, you won't see problems -- especially if you test and pass a real lab test like that. Real noise sources are generally in the tens of mV, with FCC Part 15 compliant radios topping out in the 100mW range (~30 V/m?). These days, those are usually microwave (Wifi, BlueTooth, etc.) or within an order of magnitude nearby (Zigbee, etc.?), which is fairly easy to filter, even for a capacitive touch system. The biggest hazard will be lower frequency transmissions, like proximity to licensed operators (broadcast radio, HAMs, etc.), or drive-by CB radio.Tim