AVR-Chat

NO, the internal diodes are not designed for this level of protection, 
depending on your external resistor values, you could exceed the maximum 
sink current of the device.

An external reverse-biased Schottky will provide the extra protection 
needed.

I believe in belts and braces, I would rather have a device in the field 
that does not lock up than a device that I saved a cent or two on in 
manufacturing.

Regards,

Kat.

xolang1 wrote:
>
> Appreciate the help guys. that was informative.
>  
> the datasheet indeed says the ADC's are internally diode protected.
> so then it seems the current circuit would suffice.
>  
> thanks!
>
> --- On Tue, 11/18/08, Dao Viet Dung <vietdung79@yahoo.com 
> <mailto:vietdung79%40yahoo.com>> wrote:
>
> From: Dao Viet Dung <vietdung79@yahoo.com <mailto:vietdung79%40yahoo.com>>
> Subject: Re: [AVR-Chat] Protecting the ADC
> To: AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com>
> Date: Tuesday, November 18, 2008, 4:17 PM
>
> 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@yahoogroup s.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
>
> [Non-text portions of this message have been removed]
>
> [Non-text portions of this message have been removed]
>
> [Non-text portions of this message have been removed]
>
> 
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