Re: Analog input pin protection on a MCU pin using clamping diodes...
- From: "David L. Jones" <altzone@xxxxxxxxx>
- Date: Mon, 01 Oct 2007 05:45:25 -0700
On Oct 1, 9:00 pm, Hamza <hamzasag...@xxxxxxxxxxxxxx> wrote:
Hi,
I am slightly worried about damaging the analog inputs of the
microcontroller I am using (Actually I am more concerned about other
people who like to stick random stuff at the analog inputs) and I was
wondering if something like the following circuit would give me enough
protection (so that I don't have to change the MCU every other week)
Analog
Input
o-------.-----.----------- To MCU
' '
| |
z V
3.3V Zener A - Schottky
| |
| |
=== ===
GND GND
( a nicer graphical version here:http://img258.imageshack.us/img258/281/zenerschottkywz7.png
)
I am not an expert so please bear with me while I mumble..
The 3.3V Zener would kick in if the input voltage exceeds 3.3V and
clamp it down to ground while the schottky diode prevents negative
voltages going in to the microcontroller (I am not so sure about the
direction of the schottky diode, is it connected wrong?). So the input
going to the analog channel of the mcu would be in the range of -0.3V
(because of the forward voltage drop of the schottky?) and 3.3V.
Yes, no, inefficient, horribly wrong? All comments would be much
appreciated :)
Regards,
Hamza.
Also add (or you can even use it on it's own) a series resistor, the
value of which is calculated based on the expected worse case voltage
and the maximum clamping current the chip can handle.
As always, make sure any input protection scheme doesn't affect the
desired analog performance.
Dave.
.
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