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From: rfmg on 17 Dec 2009 10:48
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?

Best Regards,
From: rfmg on 17 Dec 2009 11:08
On Dec 17, 4:58 pm, Vladimir Vassilevsky <nos...(a)nowhere.com> 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?
>
> This is what you need: TPD4E001
>
> Vladimir Vassilevsky
> DSP and Mixed Signal Design Consultanthttp://www.abvolt.com

Hi Vladimir, thanks for the reference.

However, I am thinking of a continuous overvoltage protection, neither
transient spikes or ESD. Thinks of the +3V3 data line being driven by
a +5V signal. I would like the digital transmission to still be able
to work under those condition.

In the same spirit, I would like to protect the analog line (clamp to
+3V3) even if the user attempts to measure voltage over 3V3.

Thanks,
From: Jan Panteltje on 17 Dec 2009 11:19
On a sunny day (Thu, 17 Dec 2009 07:48:38 -0800 (PST)) it happened rfmg
<francomendoza79(a)hotmail.com> wrote in
<f4fb3d38-b3dd-4269-9e33-6d39c5ae0e45(a)d10g2000yqh.googlegroups.com>:

>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?
>
>Best Regards,

The main disadvantage of zeners is the high capacitance.
The disadvantage of diodes is that, given enough current, you can lift the supply line above a safe voltage.
So if you use the diodes, or a TDO4e001 like Vladimir suggested,
then maybe you also need a circuit that prevents the 3.3V from rising too much.
That could be a high power zener, as there more capacitance is not important.

Sometimes it is also needed to have some resistors in series before the diodes or zener to limit any current.
Depends on what the data lines can make contact with, mains, 24 V DC, something else?

For very low speed data lines zeners are the simplest solutions.. or transorbs.
From: John Walliker on 17 Dec 2009 11:27
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
From: rfmg on 17 Dec 2009 11:40
On Dec 17, 5:19 pm, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote:
> On a sunny day (Thu, 17 Dec 2009 07:48:38 -0800 (PST)) it happened rfmg
> <francomendoz...(a)hotmail.com> wrote in
> <f4fb3d38-b3dd-4269-9e33-6d39c5ae0...(a)d10g2000yqh.googlegroups.com>:
>
>
>
> >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?
>
> >Best Regards,
>
> The main disadvantage of zeners is the high capacitance.
> The disadvantage of diodes is that, given enough current, you can lift the supply line above a safe voltage.
> So if you use the diodes, or a TDO4e001 like Vladimir suggested,
> then maybe you also need a circuit that prevents the 3.3V from rising too much.
> That could be a high power zener, as there more capacitance is not important.
>

Ok.

> Sometimes it is also needed to have some resistors in series before the diodes

sure,

> or zener to limit any current.
> Depends on what the data lines can make contact with, mains, 24 V DC, something else?
>
> For very low speed data lines zeners are the simplest solutions.. or transorbs.

Thanks for the info.

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