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I need to protect the ADC pin of my PIC18F27K40 from over-voltage and reverse voltage for a 0-10V input measurement.

After searching the web, I found the following solution : enter image description here

When testing with the following PIC configuration for PIN RB0: WPUB0=0 TRISB0=1 ANSELB0=1 D1 = D2 = Zener Diode 3.6V 1N4729A. I have Vrb0 between 1.4 and 1.5 V for Vin between 0 and 1.5 V.

When I take away the 2 diodes, the RB0 voltage works fine.

Can you please help me ? Am I missing something ?

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    \$\begingroup\$ you should probably use a buffer \$\endgroup\$
    – DKNguyen
    Commented Dec 3, 2019 at 14:39
  • \$\begingroup\$ You can use something like BAV99. SOT-23 single package with 2 signal switching diodes. \$\endgroup\$
    – Lundin
    Commented Dec 3, 2019 at 14:57

2 Answers 2

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You want to use regular diodes, not zeners, to clamp the ADC input voltage between GND-Vd and Vcc+Vd

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Because you are using Zener diodes they are partially conducting and the current is causing voltage drops across the resistors. Use ordinary diodes such as 1N4148s.

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