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Resistors and capacitors - pull-ups and pull-downs?

When the rise and fall time of signals is much shorter than the transit time along a trace or wire, the wave shape can be distorted by ringing if the impedance of the line does not match the source and receiver impedances. Sometimes this problem is solved by putting a series RC load on the receiving end that absorbs energy at the ringing frequency, without adding any DC load to either the logic high or low level. This also works pretty well when a low value resistor is inserted between the source and the line to slow, slightly the rise and fall times. I haven't seen a case where just a capacitor is used at either end, except for CMOS inputs that have a series resistor in the line, to produce an effective delay between signal source and downstream gate. This is a completely different function than controlling ringing and radiated noise.

Reply to
John Popelish
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I know that resistors can be used as pull-ups or pull-down, but sometimes I come across old PCBs that have capacitors linked between an input or output and ground or +5 volts. Are these caps being used to pull-up or pull-down the input or output, or for some other reason?

Thanks

Reply to
max-man

insert here: "on an IC"

:)

Reply to
max-man

snipped-for-privacy@hotmail.com (max-man) wrote in news:42f66542.207580390 @news.freeserve.net:

If on a logic IC it is most likely used as a "de-spiking" capacitor, "smoothing out" the big spike on the supply line caused by logic level shifts.

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Reply to
me

One possibility is for Electro Static Discharge protection. If one only has to protect against the Human Body Model which may consist of a 100pF capacitor at 1000 volts and a series resistance, and if the input or output line doesn't carry high frequencies, then one of the cheapest ways to protect the circuit is to put a 100nF capacitor across the line. Since the shunt capacitor has 1000 times the capacity of the Human Body Model, as it absorbs the discharge its voltage change will be only 1/1000th the discharge voltage. Note: There are many better ways of protecting against ESD but this is one of the cheapest.

Reply to
jgreimer

Many thanks for the answers guys. :-)

Reply to
max-man

More likely the caps are being used to set the inputs or lines. For example, a cap may be connected from a chip to ground, it would be safe to assume at first power on, the cap pulls that line to ground briefly until the cap is fully charged then the line behaves as normal.

Commonly used to reset chips while allowing the same line to be used during its operation.

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Reply to
Impmon

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