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| Ferrite bead/ Capacitor filter causes ringing in output? |
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| Siwastaja:
I'm seeing dips at almost exactly 1.0ms intervals - these can't be anything else but the chip drawing current in pulses, like digital ICs tend to do. Maybe your "sample interval" is configured to 1ms? Note that the current consumption is DC rating - average current. A digital IC can easily eat peaks 1000-10000x that. The most important aspect is to bypass the highest frequency components by providing a tiny bit of very low-inductance capacitance directly to the power pins - your typical 100nF cap. This tends to take care of CMOS switching, so nanosecond level stuff - but it does not filter for "bursts" of processing, your >>microsecond range. So you see the IC supply voltage fluctuating. The small bypass caps should take care so that the fluctuations are slow enough (dV/dt and dI/dt) not to cause noise coupling (EMI), but they can't "regulate" the DC bus on the mid-frequency range (tens to hundreds of kilohertz) where your typical 100n cap can't do anything, but your input power supply regulator cannot catch up, either. For this, you need a bigger capacitance (if you really need to filter it, anyway). Now, it's usually acceptable to see some noise on the digital supply lines. Is there any specific reason why you are trying to filter it? In any case, you need a steady source of charge for the chip. If you want to filter such a low-frequency bursts, the typical 100nF bypass cap isn't enough. You need way more. Try adding a large 4.7u or 10u MLCC and see what happens. (Now, adding this MLCC probably creates real conditions for the LC ringing to happen, so you may want to consider using a cap with some ~1-2ohms of ESR instead.) |
| Sjoertdb:
Thanks Siwastaja :D I've had a look in the software I2C registers of the MPR121. Indeed, as you assumed, the sample interval was set to 1 ms. :-+ :-+ :-+ The reason why the MPR121 needs a clean supply, is because the product is not properly reacting to touches. After adding a different supply and an LDO, the issue was gone. The reason of faillure was a noisy supply. I'll try adding a capacitor with some series resistor soldered into place. I'll try a 10uF one to see if the supply gets better. I really appreciate your help! I feel i'm close to the final solution. :-+ |
| Sjoertdb:
I've tried a bit of modification with a 10 uF MLCC and a 2 ohm series resistor. The scope has the following plot when the voltage is measured. It shows a delta of 146 mV, which is a little bit too high. What could be done to reduce this under a 100 mV? I've tried a bulk capacitor of 220 uF, and it reduces down to 76 mV. Is there another solution instead of such a bulky capacitor? |
| Siwastaja:
You can replace the large bulk capacitor with a regulator with a fast control loop - meaning you don't need to store as much energy, the regulator takes care of keeping the output correct, and can do it by having extra input voltage available (which is allowed to dip more). In practice, this may mean modern fast LDOs, that can easily respond in megahertz range. Most switchers can't, because the switcher's response is inherently limited by its inductor. Although, a modern high-frequency switcher should be able to do it as well. I mean, the cheapest, most integrated, modern one-chip buck running at, say, 1.5-2MHz and with a tiny inductor - and hence, relatively small capacitors as well. You shouldn't need 220uF... This single positive peak with fairly slow rising edge (100us) is bugging me. It doesn't look like inductive ringing to me, but like the device suddenly stops consuming current for a while. Can you post a schematic, or even better, an actual photo or layout picture that shows the full regulator, it's output caps, any filters you have currently in place, and finally, the bypass capacitors for the load? |
| Sjoertdb:
The schematic of the 3V3 is as follows (in the attachment). I also have a 5V line coming in, i should be able to add an LDO to that line with a very fast response time. Or accept the 200 mV drop when i put it to the 3V3 line. |
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