I am building a wireless charger based on bq500212A (datasheet here)

However, I have an issue with the LDO: It fluctuates the voltage by about 400mV in a wave.

Blue is where I put my oscilloscope probe. Green is where I put the oscilloscope GND. Green squares represent the installed components. enter image description here

Here are some oscilloscope images:

enter image description here enter image description here enter image description here

Added AC coupling: enter image description here

It's my first time using the oscilloscope for this purpose. This is what I could find on the internet. I also checked the 5V line and it does not present these fluctuations.

Could you tell me what am I doing wrong and why I don't get a stable 3V3 output?

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    \$\begingroup\$ As you want to design a Wireless charger, there will be lots of EMC Noise. You should use the SMD DSO probe, otherwise, it will pick lots of noise, which is happening with you. Follow the link "i2.wp.com/circuitcellar.com/wp-content/uploads/2014/02/…" \$\endgroup\$ – Deepak Oct 5 '20 at 18:39
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    \$\begingroup\$ What is your source of power, please also check noise on the power input \$\endgroup\$ – Deepak Oct 5 '20 at 18:48
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    \$\begingroup\$ I can see that you are using the Enable pin, but I do not see what pulls to a high level when the LDO is enabled. \$\endgroup\$ – csabahu Oct 5 '20 at 20:15
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    \$\begingroup\$ This is a bit strange for oscillation, but if it is, its frequency will change if you connect one in parallel with the output capacitor. \$\endgroup\$ – csabahu Oct 5 '20 at 20:25
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    \$\begingroup\$ The stability of the LDO requires a capacitor at its output. From what I see to the right of it, I thought it was a capacitor. \$\endgroup\$ – csabahu Oct 5 '20 at 20:36

This seemed to be a problem with the value of the capacitor I was using after the LDO.

Below are some screenshots:

This is the LDO, using a ~4uF capacitor. Note the dips 300mV (which is HUUUGE) enter image description here

And this is the LDO with the right 100nF, as pointed in the datasheet here: enter image description here

This is probably the clearest 3.3V source I have seen so far. Note how it sits within 10mV.

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    \$\begingroup\$ You worked a lot with the power supply, but I like that you even had to read the datasheet in the end. :) \$\endgroup\$ – csabahu Oct 7 '20 at 5:12

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