Re: [SI-LIST] : Driver Strength

Lfresearch (Lfresearch@aol.com)
Thu, 5 Mar 1998 13:57:39 EST

Chaps,

the discussion on driver strength has prompted me to make an observation seen
during EMC testing. One of the immunity tests fires rapid ( ns timeframe )
voltage transients onto all the wiring interfaces. I have observed that when
digital circuits respond to the transients, it is the devices with the weakest
pull-up or pull-down that go first. By reversing the transient polarity,
significant shifts in the level at which the circuit responds may be observed.
Also, if the pulling circuit is not located close to the input pin, the trace
isolates its effect somewhat.

The problem is that by using drivers with lower drive capability we end up
with designs that could have immunity problems. However, devices with higher
drive capability tend to be the cause of radiated emissions problems.

I guess there needs to be middle ground ( from the EMC engineers point of view
), so that we can maximise immunity and minimise emissions. Anyone want to
take a shot at a set of guidelines?

Have a super day,

Derek Walton.
Owner L F Research EMC Design and Test Facility