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Hello

I would like to know if on the analog input pin there are diode protection
to prevent overvoltage?

If yes are they connected to VDD or VSS or both?

Thank you very much

Regards

_________________________________________________________________________

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Par precaution, n'ouvrez pas de pieces jointes de correspondants inconnus.
_________________________________________________________________________

[Non-text portions of this message have been removed]
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(You need to be a member of 68hc12 -- send a blank email to 68hc12-subscribe@yahoogroups.com )

"pascal DEREX Liberty" wrote:

> Hello
>
> I would like to know if on the analog input pin there are diode protection
> to prevent overvoltage?
>
Yes, there are protection diodes, even on analog inputs.
> If yes are they connected to VDD or VSS or both?
>

They are connected to both VDDA and VSSA. Anywy you should stay within
supply rails and try not to make internal A/D input protection diodes
conducting any current (current injection). There was some document
explaining that current injection to any ATD pin introduces A/D errors not
only on pin with out of specs voltage, but also on other A/D pins.

Edward

> Thank you very much
>
> Regards
>
> _________________________________________________________________________
>
> Ce message a ete verifie par l'antivirus de MDaemon (md6).
>
> Par precaution, n'ouvrez pas de pieces jointes de correspondants inconnus.
> _________________________________________________________________________
>
> [Non-text portions of this message have been removed]
> ------------------------------------



(You need to be a member of 68hc12 -- send a blank email to 68hc12-subscribe@yahoogroups.com )

Thank you Edward,

=20

Ok there are input protection diode. What is the best practice in order to
protect A/D input pin in case of overvoltage (for example components error
when building the card) should we add a protection diode (zener for example=
)
on the input pin?

=20

Regards

=20

=20

De : 6...@yahoogroups.com [mailto:6...@yahoogroups.com] De la part de
Edward Karpicz
Envoy=E9 : jeudi 24 avril 2008 06:09
=C0 : 6...@yahoogroups.com
Objet : Re: [68HC12] Analog converter protection

=20

"pascal DEREX Liberty" wrote:

> Hello
>
> I would like to know if on the analog input pin there are diode protectio=
n
> to prevent overvoltage?
>

Yes, there are protection diodes, even on analog inputs.

> If yes are they connected to VDD or VSS or both?
>

They are connected to both VDDA and VSSA. Anywy you should stay within=20
supply rails and try not to make internal A/D input protection diodes=20
conducting any current (current injection). There was some document=20
explaining that current injection to any ATD pin introduces A/D errors not=
=20
only on pin with out of specs voltage, but also on other A/D pins.

Edward

> Thank you very much
>
> Regards
>
> __________________________________________________________
>
> Ce message a ete verifie par l'antivirus de MDaemon (md6).
>
> Par precaution, n'ouvrez pas de pieces jointes de correspondants inconnus=
.
> __________________________________________________________
>
> [Non-text portions of this message have been removed]
> ------------------------------------



(You need to be a member of 68hc12 -- send a blank email to 68hc12-subscribe@yahoogroups.com )

I usually do like this for protecting circuits connected to external
components:

Terminal - Emi filter - resistor - tranzorb - resistor - ic pin

Emi filter both blocks noise from the chip and to the chip. Should be connected
to a good groundplane close to the terminal.

Resistors are low ohm (a couple of ohms to 100 ohms) resistors that dissipates
any energy in case of a spike before it reaches the ic. These should be a type
that is capable of withstanding a short surge.

The tranzorb has a knee voltage around the absolute maximum for the ic. Note
that a tranzorb can have a pretty high capacitance though (a couple of nano
farads).

/Ruben


> Thank you Edward,
>
> Ok there are input protection diode. What is the best practice in order to
> protect A/D input pin in case of overvoltage (for example components error
> when building the card) should we add a protection diode (zener for example) on
> the input pin?
>
> Regards
>
> De : 6...@yahoogroups.com [mailto:6...@yahoogroups.com] De la part de
> Edward Karpicz
> Envoyé : jeudi 24 avril 2008 06:09
> À : 6...@yahoogroups.com
> Objet : Re: [68HC12] Analog converter protection
>
> "pascal DEREX Liberty" wrote:
>
> > Hello
> >
> >
> >
> > I would like to know if on the analog input pin there are diode protection to
> > prevent overvoltage?
> > Yes, there are protection diodes, even on analog inputs.
>
> > If yes are they connected to VDD or VSS or both?
> > They are connected to both VDDA and VSSA. Anywy you should stay within
> supply rails and try not to make internal A/D input protection diodes
> conducting any current (current injection). There was some document
> explaining that current injection to any ATD pin introduces A/D errors not only
> on pin with out of specs voltage, but also on other A/D pins.
>
> Edward
>
> >
> >
> > Thank you very much
> >
> >
> >
> > Regards
> >
> >
> >
> >
> >
> > __________________________________________________________
> >
> > Ce message a ete verifie par l'antivirus de MDaemon (md6).
> >
> > Par precaution, n'ouvrez pas de pieces jointes de correspondants inconnus.
> > __________________________________________________________
> >
> > [Non-text portions of this message have been removed]
> >
> >
> > ------------------------------------
> >
> >



(You need to be a member of 68hc12 -- send a blank email to 68hc12-subscribe@yahoogroups.com )

>
> Ok there are input protection diode. What is the best
> practice in order to
> protect A/D input pin in case of overvoltage (for example
> components error
> when building the card) should we add a protection diode
> (zener for example)
> on the input pin?
>

An alternative could be connect the input terminal via a relatively high
resistor (10K) R1 to the + input of a rail-to-rail op amp in voltage
follower config. Also cheap protection diodes 1N914 or similar from the +
input to VCC and GND.

Then power the op amp with same VCC as chip, this will guarantee that the
input voltage to chip will remain inside VCC-GND.

The high input resistance 10K will protect the circuit from overvoltage. The
voltage follower op amp is needed because otherwise the ADC input impedance
(relatively low) would cause reading errors.

Advantages: No expensive protection parts, no loss of precision caused by
leakage through zener diodes etc

Possible drawback: Some rail-to-rail op amps may have problems with voltages
close to the rails but that can sometimes be fixed using pullup or pulldown
resistors

You could also add a small c from diode-diode junction to create lowpass
filter. Also perhaps add 10K from the R1-diode-diode connection to the +
input of op amp to make the circuit even more bullet proof (following the
principle design and forget :)

Anders
------------------------------------



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Thank you for yours answers

REgards

_________________________________________________________________________

Ce message a ete verifie par l'antivirus de MDaemon (md6).

Par precaution, n'ouvrez pas de pieces jointes de correspondants inconnus.
_________________________________________________________________________

[Non-text portions of this message have been removed]
------------------------------------



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