AVR-Chat

On Nov 17, 2008, at 9:19 PM, xolang1 wrote:

> Hi
>
> I have an ATmega128 utilizing the ADC to read voltage from a
> thin film vibration sensor that generates voltages from 0 - 70V.
>
> Im currently using a simple resitor voltage divider to scale down
> this 0-70V to 0-5V. To protect the ADC pin, i have a 5V Zener
> across the ADC input.
>
> Question: is this set-up sufficiently protected?
> any suggestions?
>
> Thanks in advance for the help.
>
> chris
>
>
> 
A zener is not a particularly good choice. It is too "soft" at its  
breakdown. Further, if it is really rated at 5.0V, it will have some  
tolerance (probably 10% for an ordinary zener) and this could make the  
breakdown as low as 4.5V. I would use a small reverse-biased Schottky  
diode from the ADC input to Vcc.  Let the source resistance of the  
divider limit the current. This will keep the input from exceeding the  
max of Vcc+0.3V that is a common spec.

Jim Wagner
Oregon Research Electronics

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