AVR-Chat

Well, if you are military and making something to survive a nuclear EMP
event, you can do it: sealed metal box, with EMI filters on all lines going
into/out of the box (choke, capacitor, clamp diodes, resistors).  That is
about as idiot proof as possible (and very expensive).

For me, like Roy, below, 1-100k resisters and letting the input protection
diodes handle the load works well.  For analog inputs it is a bit trickier.
You need to understand how they work and the signal you are dealing with,
but again, for me the 1-10k + .01uf works great.

For digital signals, the input impedance is very high (100's megohms) so
100k is not that serious.  If you are using pull-up resisters, of course,
they have to be external (can't use the internal 50-100k resistors in the
AVR).  Even a 10kv discharge will only dump 100ma max through the input
protection diodes and I believe that most modern CMOS inputs are latch-up
proof.  In any case, a 1kv discharge is only 10ma and that is safe for any
but the most archaic semiconductors.  The story gets better if there is a
bit of capacitance on the input as the usual source of static discharge is
pretty low capacitance (1000pf?) so the spike gets smoothed out and the
current spread out for a lower overall peak value.

For analog inputs, Atmel recommends < 5k source impedance for accurate
results.  So, if you use > 5k input protection, then you need some
capacitance to lower the effective impedance at the pin.  That bandwidth
limits the signal.  For anything I do that is never an issue as my sample
rates are rarely over 1-4 kHz.

Cheers!

-----------
Larry Barello
www.barello.net


| -----Original Message-----
| From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
| Of Roy E. Burrage
| Sent: Saturday, July 08, 2006 10:45 PM
| To: AVR-Chat@yahoogroups.com
| Subject: Re: [AVR-Chat] Re: Circuit for protecting analog inputs?
| 
| You might use a diode in series with the power input, Ryan, to protect
| from polarity inversion.  Better yet, use a bridge rectifier and you can
| use either polarity DC as well as AC input from a wall wart.  That way
| all you do is connect power...no problems with polarity or type as long
| as the voltage limits aren't exceeded.
| 
| For inputs, we use a 10K resistor in series with the input and depend on
| the internal diodes for clamping.  If speed isn't a factor, we might use
| 2 resistors and a 0.1uf or a 0.01uf cap between them to common for a
| noise filter...which isn't a bad idea any time you can do it.
| 
| You can only go so far in making things idiot proof, you know.
| 
| 
| REB
| 
| 
| 
| Ryan wrote:
| 
| >Thanks, I know exactly what you're talking about.  However, for
| >someone who might inadvertantly connect a +12V supply to the pin... It
| >would obviously try to sink lots of current through the diode, likely
| >blowing it up, then blowing up the MCU.  Would the effectiveness be
| >reduced by putting series resistors (say, 1k?) with the two diodes?
| >
| >Or if the supply was connected backwards, all the current would try to
| >go through both diodes....  Trying to create an idiot proof box, but
| >you know how that goes.  ;)
| >
| >Ryan
| >
| >
| >
| >--- In AVR-Chat@yahoogroups.com, Ken Arck <ah6le@...> wrote:
| >
| >
| >>At 10:38 PM 7/8/2006 -0000, you wrote:
| >>
| >>I need to protect an analog input for an AVR.  Protect it from ESD,
| >>users connecting it wrong (i.e. to 12V power), etc.  Any suggestions
| >>on what to use?  Unity gain opamp?  "transorb" or similar transient
| >>device???  Discussion welcome.  Thanks,
| >>
| >><---Some expouse using zeners and currentl limiting resistors but I've
| >>always found the leakage current to be an issue. So now I use regular
| >>diodes to ground and the Vcc rail. See the attached schematic. The
| >>
| >>
| >diodes
| >
| >
| >>are general purpose silicon like 1N914 or 1N4148.
| >>
| >>Ken
| >>
| >>  ----------
| >>
| >>
| >>
| >>
| >>
| >-------------------------------------------------------------------------
| -----
| >
| >
| >>President and CTO - Arcom Communications
| >>Makers of the world famous RC210 Repeater Controller and accessories.
| >>http://www.ah6le.net/arcom/index.html
| >>Authorized Dealers for Kenwood and Telewave and
| >>we offer complete repeater packages!
| >>AH6LE/R - IRLP Node 3000
| >>http://www.irlp.net
| >>
| >>[Non-text portions of this message have been removed]
| >>
| >>
| >>
| >
| >
| >
| >
| >
| >
| >
| >
| >Yahoo! Groups Links
| >
| >
| >
| >
| >
| >
| >
| >
| >
| >
| 
| 
| [Non-text portions of this message have been removed]
| 
| 
| 
| 
| 
| Yahoo! Groups Links
| 
| 
| 
| 
| 
|