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From: John Larkin on 17 Dec 2009 18:24
On Thu, 17 Dec 2009 08:40:34 -0800 (PST), rfmg
<francomendoza79(a)hotmail.com> wrote:

>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.

A pair of depletion-mode fets, like LND150s, in series with an input
will limit the current to about 1 mA up to hundreds of volts of
overload. After that, most chip ESD diodes can handle it.

John

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