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

M

max-man

Jan 1, 1970
0
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
 
J

John Popelish

Jan 1, 1970
0
max-man said:
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?

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.
 
J

jgreimer

Jan 1, 1970
0
John Popelish said:
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.

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.
 
M

me

Jan 1, 1970
0
[email protected] (max-man) wrote in @news.freeserve.net:
insert here: "on an IC"


:)

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.
 
I

Impmon

Jan 1, 1970
0
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?

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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