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For example I'd like to apply AC voltage ~3V. Could I not detect, but simply connect to MCU? I'd measure positive side as regular ADC, but I simply do not want negative side of amplitude to damage it. Would 100k Ohm resistor in series prevent such damage?

Below is analog input model from datasheet of MCU:

enter image description here

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  • \$\begingroup\$ Ri Di - Hi, To comply with the site rule on referencing, please edit the question & add the PDF / video / webpage name and its link for the source of that copied image. (And please remember it's your responsibility to follow that rule in future). TY \$\endgroup\$
    – SamGibson
    Feb 1 at 19:12
  • \$\begingroup\$ Although using a 100k resistor in series with the input probably would prevent damage, you'd be violating the recommendation shown in the image you posted in your question, that Rs be less than 10k. \$\endgroup\$
    – brhans
    Feb 2 at 14:54
  • \$\begingroup\$ Could this limitation be because of speed - i.e. if someone would want to detect voltage change quickly, it would be too slow? \$\endgroup\$
    – Ri Di
    Feb 2 at 18:13

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Would 100k Ohm resistor in series prevent such damage?

3V AC is 4.2Vpp. 4.2V through 100kΩ to ground is 42μA - well below recommended or maximum input current ratings of pretty much any MCU's input pin.

So, yes, it will prevent damage from overcurrent and will prevent latchup.

If anything, you'll need to check if the RC time constant of the 100kΩ resistor in series with the input pin capacitance won't filter the signal out too much.

Suppose the input capacitance is 20pF - a ballpark value. \$20{\rm\,pF}\cdot100{\rm\,k\Omega}=2{\rm\,\mu s}\$. The low-pass cutoff frequency is \$1/(2{\rm\,\mu s})=500{\rm\,kHz}\$. If the AC signal is higher than that in frequency, it may end up too small to go past the input threshold and register as a logic 1 level.

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  • \$\begingroup\$ Thanks! I did not know about input capacitance effect, so I checked - PIC has 5pF. I edited the question with analog input schematic. Would the information displayed there change something? Especially the 'Leakage 500nA'? \$\endgroup\$
    – Ri Di
    Feb 1 at 18:55

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