[sdiy] Digital Input Circuit

[cheater00 .]

On Thu, Aug 8, 2013 at 5:28 PM, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> Tim Ressel wrote:
>>>> I use a current limiting resistor (2K) followed by a BAS40-04 dual schottky diode which acts as a clamp, then into a 74HC14 schmitt trigger for cleaning up the edges. This arrangement can handle +/- 15 volts on the inputs without damage. Also good for analog inputs, except the HC14 turns into an opamp buffer.
>
> cheater00 wrote:
>>>In a parallel thread on TekScopes, TVS diodes were recommended.
>
> Tim Ressel wrote:
>> TVS diodes have a higher
>> voltage. Schottky diodes have a 0.3 volt drop when conducting. This
>> means the inputs to the processor do not exceed specifications.
>
> TVS diodes are good for absorbing ESD spikes, and you generally want
> them as close to the signal input as possible to minimise the
> radiating loop, sometimes with a low-value series damping resistor.
> But they'll clamp the spike at a highish voltage.  A higher-value
> series resistor and Schottky diodes as Tim suggests then protects the
> internal gate inputs.  And use Schottkys rather than ordinary silicon
> diodes or you won't know whether the specific diode you soldered down
> (good) or the IC's internal diode (bad) will conduct first.

Are you saying one should use TVS and then Schottky?

Cheers,
D.