AVR-Chat

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> wrote:

> From: Kathy Quinlan <kaqdialup@iinet.net.au>
> Subject: Re: [AVR-Chat] Protecting the ADC
> To: AVR-Chat@yahoogroups.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>> 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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