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Per the title, we have a 7" TFT with a Goodix GT911 touchscreen controller which connects on a small FPC (power, ground, I2C and RST/INT lines), however when the touch controller is operational we see hefty bursts of noise on the 3v3 supply rail despite it being fairly well decoupled right on the FPC connector with 100nF + 10uF MLCC's, a more than adequate power supply rail (AP7366 rated 600mA continuous - GT911 claims ~8mA consumption) with a thick PCB trace, and good solid grounding right around the connector with multiple vias down to the ground plane.

The noise appears to have a 50 kHz main component and happens in bursts which I assume correspond to scans of the touch area(s).

I can find nothing in the datasheet / design guide or general internet searching about this sort of thing being a problem, so was wondering if anyone here had experienced similar issues or had any useful info or app notes or suchlike?

Scope trace showing AC component of 3.3 V rail measured right on FPC connector next to decoupling/smoothing caps:

Power supply trace

Before you say it, yes I know the Rigol scope is fairly noisy by default.

  • Scope probe is standard X10 probe, probing on the decoupling capacitors which are only millimetres from the FPC connector, ground lead to PCB ground pour which is all round the connector.
  • FPC is 10-way 0.5mm pitch, wired GND, VDD, SCL, GND, SDA, GND, NRST, NC, INT, GND. GT911 controller plus associated passives are on this FPC.
  • PCB is 4-layer, FPC grounds come out of connector into ground pour, with multiple vias down to ground plane below.
  • Snippet of PCB below showing connector, cap, grounding, and scope probe points.
  • R157 + R158 are the I2C pullups, 4.7 k.
  • C94 was 100nF MLCC, then I added a 10uF MLCC on top of it (stacked 0603's). Adding a 47 uF electrolytic across the pads of C94 had no appreciable effect. I can't say for sure this is a problem but the touchscreens are sometimes causing problems (I2C lockups) which is what led me here, and the spikes feel unwelcome at the very least. GT911 specifies <=50 mVpp supply ripple so we're borderline.
  • I added 2x 100R in parallel (to increase the load) but no difference - the AP7366 is stable with zero load and it's a linear LDO which doesn't have different modes of operation.
  • I made a pigtail and re-measured it right across the ends of C94 and it made no difference at all to the trace.

Connector & probe illustration

I added a 47uF electrolytic on the rail, it reduced the amplitude by a few millivolts and that's about it.

Here's a zoomed-in trace of noise burst:

Close up of noise

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    \$\begingroup\$ (To be clear, it's mostly in the 10s of MHz -- "HF" as such -- where probe grounds make a big difference. It would be difficult for this to occur at 50kHz; one would need a serious ground loop elsewhere to do that. Which is still possible, but more obvious to spot, and less likely to happen.) \$\endgroup\$ Dec 15, 2022 at 12:12
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    \$\begingroup\$ @TimWilliams Yep, the 1 ms time scale was deterring but some scope (settings) will happily fold over (not sure on the English term here) a HF signal on long time base. Not the case here. \$\endgroup\$
    – winny
    Dec 15, 2022 at 13:21
  • \$\begingroup\$ @winny I think it's called aliasing, where a HF signal looks like a much slower one. But I had checked it on a faster time-base before my original post, I just thought the longer view gave a better impression of the overall signal (bursts of noise) rather than a specific capture of the 50Khz waveform. \$\endgroup\$
    – John U
    Dec 15, 2022 at 14:43
  • \$\begingroup\$ This conversation has been moved to chat for further discussion. \$\endgroup\$
    – Null
    Dec 15, 2022 at 15:45

1 Answer 1

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50 kHz is in that range which regulators don't regulate very well but where small MLCC do not yet provide a low power supply impedance.

The 10 uF cap has 0.3 ohm impedance (likely more when considering DC bias derating), so for the supply to ripple 50 mV, current would need to be 150 mA during the burst. I guess that sounds reasonable with an average current of 8 mA.

If you don't have bulk capacitors, add one or several. It will also help to damp potential LC resonance of the MLCC with the long feed line to the screen.

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  • \$\begingroup\$ I stuck a 47uF electrolytic on the rail but it made no real difference, even adding a 2nd one right on the cap by the connector (C94). \$\endgroup\$
    – John U
    Dec 15, 2022 at 10:50
  • \$\begingroup\$ 47uF electrolytic caps usually have more than 1 ohm ESR, so may not help much indeed. If it helped a little bit, then go with something bigger like 330 uF. If it didn't change anything, it might be a problem with how you scope (inductive pickup) \$\endgroup\$
    – tobalt
    Dec 15, 2022 at 11:11
  • \$\begingroup\$ @JohnU do you have a part number for the cap? \$\endgroup\$ Dec 16, 2022 at 3:20
  • \$\begingroup\$ Multicomp MCESL50V476M6.3X7.7 \$\endgroup\$
    – John U
    Dec 19, 2022 at 8:53
  • \$\begingroup\$ @JohnU The datasheet doesn't list 100kHz ESR as some manufacturers do. Only 120 Hz. That usually means, that this cap is on the high end of ESR specs. I would assume ~1.5-3 Ω. That would also explain why it doesn't do anything at 50 kHz next to the 10 µF MLCC. \$\endgroup\$
    – tobalt
    Dec 19, 2022 at 9:00

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