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From: rfmg on 17 Dec 2009 11:41
On Dec 17, 5:27 pm, John Walliker <jrwalli...(a)gmail.com> wrote:
> On 17 Dec, 15:48, rfmg <francomendoz...(a)hotmail.com> wrote:
>
> > voltage analog and data lines. I am specially interested in advantages/
> > disadvantages of a zener diode connected between a data line and GND
> > to clamp voltages to 3V3 compared to two standard diodes: one
> > connected between data and GND and the other connected between data
> > and 3V3 (both inversely polarized under normal conditions). Speed?
> > Price?
>
> A suitable pair of diodes will have much lower capacitance than a
> zener diode and will therefore have less effect on the wanted signal.
> Don't forget to include some series resistance to limit the fault
> current to a safe value for the expected overvoltage.   BAV99 double
> diodes are good enough for most purposes and are cheap.
>
> Clamping to the 3V3 supply needs caution, as the overvoltage signal
> can easily cause the supply to rise in voltage and damage components
> by that route.  If you are just protecting against electrostatic
> discharge, the power supply decoupling capacitors will absorb the
> extra charge, but accidental connection to a 12V power supply, for
> example, could cause problems.
>
> To avoid this issue, connect the upper diode to a 3V zener diode the
> other end of which is grounded.  This prevents the fault current from
> getting into any of the active circuits. It is a good idea to prebias
> the zener with a resistor to the 3V3 supply.  This ensures that the
> wanted signal does not have to charge up the zener at startup which
> might otherwise cause momentary distortion.
>
> John

Thanks for the info, it is just what wanted to hear.
From: rfmg on 20 Dec 2009 06:36
On Dec 18, 1:20 am, Joerg <inva...(a)invalid.invalid> wrote:
> rfmg wrote:
> > Hi guys,
>
> > I am working on a design where I have a couple of analog (ADC) lines
> > measuring a range between 0V and 3V3, and a couple of digital data
> > lines transmitting/receiving  0V and 3V3 voltage levels, and I want to
> > protect them from overvoltage.
>
> > Hence, I was wondering if you have any reference to read about
> > different configurations for overvoltage protection of low-level-
> > voltage analog and data lines. I am specially interested in advantages/
> > disadvantages of a zener diode connected between a data line and GND
> > to clamp voltages to 3V3 compared to two standard diodes: one
> > connected between data and GND and the other connected between data
> > and 3V3 (both inversely polarized under normal conditions). Speed?
> > Price?
>
> A zener diode is not very precise and a 3V3 is also quite soft. I
> usually go for a BAV99 against VCC and GND or if leakage is a concern a
> BAV199. Then a small resistor between that and the chip pin so that the
> chip has no chance to get hit with the lion's share of the spike current
> through its substrate diode. A resistor on the outside is also best but
> you may not be able to do that for ports that have to drive long data lines.
>
> With diodes you must also watch the rail. If your crcuitry consumes
> little power and you experience a lot of ESD hits these may drive up
> your rail. This is because nearly all regulators, whether switch-mode or
> linear, can only source current but not sink any. A TL431 is often
> enough to remedy this.
>
> --
> Regards, Joerg
>
> http://www.analogconsultants.com/
>
> "gmail" domain blocked because of excessive spam.
> Use another domain or send PM.

Thanks for the diodes' suggestion.
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