Hi guys, I've been reading some datasheets just practicing extracting the useful data and thinking about somethings previously mentioned in this section. I was wondering if u can give an explanation on these terms or point me to a site which will explain:

1. EMI (Seems to come from high speed [ > ~25Mhz] microprocessors)

2. Decoupling (I understand decoupling capacitors are needed to stabilize the power supply voltage but I am the type that likes to understand concepts, especially when it comes to electronics/computers).

3. Also I noticed that some digital IC pins have a capacitance rating on them? Does that mean charge actually accumulates on the pin? Does it affect the devices response time ? (I'm assuming that is the case)

Anyways thanks for your help guys :D I gotta go back and study for exams ( I HAVE 4 IN ONE SEMESTER ARRRRGGGGGHGHGHGHGG >=| )
Posted on 2003-06-14 14:50:58 by x86asm
1) EMI: Stands for Electromagnetic Interfearance. A catch-all term for unintended results, like when your egg beater makes lines on your TV set. Computers are a rich source of EMI, as the fast switching speeds results in lots of RF energy being emitted. Usually required an expensive lab to fully measure and evaluate.

2) Decoupling: For logic ICs, the current in and out of the pins has to come from somewhere. As the power supply is usually quite distant, the power line at the IC actually dips and peaks in value as the output transistions. The worst cause is the series inductance of the power supply lines.

To overcome this, a small capacitor is placed direct on the power lines at the IC. This gives a low impeadancce local source for the transient currents.

3) Pin Capacitance: A real capacitance assosciated with a pin, comes from both the packaging (it's metal after all, and two pieces of metal form a cap), and the internal circuit of the IC. Espically in CMOS (where pin capacitance is the major load) but in other logic families too, care must be taken to drive an inout with suficcient current to charge and discharge this cap.
Posted on 2003-06-15 17:28:39 by Ernie