Re: Analog input pin protection on a MCU pin using clamping diodes...

On Oct 1, 1:25 pm, NoS...@xxxxxxxxxxx (Bob Masta) wrote:
On Mon, 01 Oct 2007 04:00:18 -0700, 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 common clamping arrangement (used inside
of many chips) is to connect reverse-biased ordinary
diodes to the rails at the input, and feed that point
from the outside world through a current-limiting resistor.
The diode from the input to the positive rail will thus
do nothing until the input tries to exceed the rail by
0.6 V, and anything above that will be clamped.
Same idea for the negative rail (or ground).
The value of the current-limit resistor must be
large enough to protect the diodes under the
worst anticipated input, and small enough to
not compromise the input signal due to the
low-pass filter formed with the chip's intrinsic input
capacitance.

Best regards,

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
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- Show quoted text -


Thank you all very much for your input.

After looking around for a bit, I came across the MAX3208E, which is a
"low-capacitance, ±15kV ESD-protection diode arrays with an integrated
transient voltage suppressor (TVS) clamp are suitable for high-speed
and general-signal ESD protection.", according to the data***.
Instead of using lots and lots of diodes on the board I'd prefer to
use something like this, maybe with an added resistor.

The following screenshot from the data*** shows how it is internally
connected: http://img252.imageshack.us/img252/1659/max3208ol5.png

After reading Traver's comment on using an op-amp as a buffer I've
read a bit more on it and it looks like a safer solution than using
the diodes to do the clamping, for the reasons he has stated but I am
not convinced that it would be able to cope with the input voltages
exceeding the op-amp's rail voltage. Can the op-amp withstand huge ESD
transients or say 60V on a 3.3V power rail?


Thanks again for all your comments.


Kind Regards,
Hamza.

.