How to protect an analogue input

[Steve001]

Evening All

Looking to protect against an over voltage to an analogue input pin, to prevent against those "accidents when rushing " and doing warp factor 10

have been thinking of the following

1. series resistor(100 R) with zener diode (5V1) The input pin.
2. op amp wired as a voltage follower with a 5 volt single rail supply
3.https://www.maximintegrated.com/en/prod ... AX366.html - Fig 2.

Any ideas / suggestions / recomendations

Steve

[medelec35]

Since the i/p has diode protection (except VPP) and internal resistance is high, I use a 10k resistor in series with source & i/p that's it.

[Steve001]

Hi Martin,

would this protect the input if it went well above 5 volts, if the input was incorrectly selected.
Do the analogue inputs have diodes to vcc and vss internally

Steve

[medelec35]

Hi Steve,

would this protect the input if it went well above 5 volts

Yes it would.
Looking at the data sheet of 16F1937 for example, it states:

Code: Select all

Clamp current, IK (VPIN < 0 or VPIN > VDD)................................................................................................................± 20 mA

So if voltage is 5+0.7V then with 10K the very maximum voltage on the i/p will be:
20E-3*10E3+5.7 = 205.7V
It's best not to go anywhere near that voltage if you can help it.
If it was me I would probably use a much safer figure of say 700uA
So max voltage = 700E-6*10E3+5.7 = 12.7V
What maximum voltage did you have in mind?

Do the analogue inputs have diodes to vcc and vss internally

Yes the diodes are built in.
The datasheet should show this.
Again looking at 16F1937:

Built in ESD diode protection.png

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Martin