AVR-Chat

All of the ATmega series themself have protected diode inside. So you should find the other way to protect it.

--- On Tue, 11/18/08, Jim Wagner <wagnerj@proaxis.com> wrote:
From: Jim Wagner <wagnerj@proaxis.com>
Subject: Re: [AVR-Chat] Protecting the ADC
To: AVR-Chat@yahoogroups.com
Date: Tuesday, November 18, 2008, 12:30 PM










    
            

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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