AVR-Chat

By doing it that way you can also protect from reverse spikes, which is 
a great idea.



xolang1 wrote:
>
>
>
> the source resistance should be around 50-75k
> which should be sufficient for current limiting.
> for over voltage protection, reverse Ge diodes (0.3V) will be
> placed from adc to vcc and gnd to adc.
> any thing missed or inappropriate?
>
> --- On Tue, 11/18/08, Kathy Quinlan <kaqdialup@iinet.net.au 
> <mailto:kaqdialup%40iinet.net.au>> wrote:
>
> > From: Kathy Quinlan <kaqdialup@iinet.net.au 
> <mailto:kaqdialup%40iinet.net.au>>
> > Subject: Re: [AVR-Chat] Protecting the ADC
> > To: AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com>
> > Date: Tuesday, November 18, 2008, 8:11 PM
> > 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>
> > > <mailto:vietdung79%40yahoo.com>> wrote:
> > >
> > > From: Dao Viet Dung <vietdung79@yahoo.com 
> <mailto:vietdung79%40yahoo.com>
> > <mailto:vietdung79%40yahoo.com>>
> > > Subject: Re: [AVR-Chat] Protecting the ADC
> > > To: AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.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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>
> 
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