AVR-Chat

I have an application where I need to use an ATmega8 port pin to apply 
bias current to a PIN diode RF switch.  10ma is all that's needed, which 
is no problem.  However, the side of the RF switch that is being 
supplied current is connected to a mobile vehicle receiving antenna.  My 
concern is that static build up on the antenna whip during motion will 
pose an ESD risk to the ATmega8.

In practice, the antenna will have a DC resistance to ground of only a 
few hundred ohms, so I'm certain this will be enough to drain off any 
long-term accumulation, but is this sufficient?

I know the pins have ESD protection diodes.  I've never given them much 
thought before so I don't know if how effective they are at dealing with 
this sort of environment.

I'm probably being overly careful.  I suppose I could put a buffer 
between the I/O pin and the RF switch bias input, so the (cheap) buffer 
would get toasted before the (more expensive to repair) ATmega8.  But is 
this even worth worrying about?

-Johnathan

Johnathan,

ESD protective devices (wrist straps, mats, chairs, etc.) have a 1M 
resistor in the drain wire to bleed off the charge built up in a work 
area.  I've seen these work well even for women wearing pantyhose and 
polyester clothing...not that we really want to test them to that extent 
all the time, they do work.  With this in mind, if you have a few 
hundred ohms in parallel with your output you'll probably be okay.

The only time I ever saw a problem with the 1M resistors was when a 
customer had their ESD grounding system connected to the same earth 
ground as the machinery on the production line.  When they had a motor 
short to ground the ladies on the assembly line got a bit excited...and 
they ran a separate ground line the same morning.  It wasn't enough to 
do harm to them but they were able to feel a tingle.

If you're going to do a lot of work with ESD as a major concern, it 
might be wise to invest in an electrostatic voltmeter.  A butane 
charcoal lighter can also make a good ESD generator...but drain the 
butane first.


REB


Johnathan Corgan wrote:

>I have an application where I need to use an ATmega8 port pin to apply 
>bias current to a PIN diode RF switch.  10ma is all that's needed, which 
>is no problem.  However, the side of the RF switch that is being 
>supplied current is connected to a mobile vehicle receiving antenna.  My 
>concern is that static build up on the antenna whip during motion will 
>pose an ESD risk to the ATmega8.
>
>In practice, the antenna will have a DC resistance to ground of only a 
>few hundred ohms, so I'm certain this will be enough to drain off any 
>long-term accumulation, but is this sufficient?
>
>I know the pins have ESD protection diodes.  I've never given them much 
>thought before so I don't know if how effective they are at dealing with 
>this sort of environment.
>
>I'm probably being overly careful.  I suppose I could put a buffer 
>between the I/O pin and the RF switch bias input, so the (cheap) buffer 
>would get toasted before the (more expensive to repair) ATmega8.  But is 
>this even worth worrying about?
>
>-Johnathan
>  
>