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ac coupling

G

Grumps

Jan 1, 1970
0
Hi
When laying out a pcb/pwb, if a signal source needs to be ac coupled to a
receiver, is it best to:
1) put the cap nearest the source,
2) in the middle, or
3) nearest the receiver?
 
A

Andrew Holme

Jan 1, 1970
0
Grumps said:
Hi
When laying out a pcb/pwb, if a signal source needs to be ac coupled to a
receiver, is it best to:
1) put the cap nearest the source,
2) in the middle, or
3) nearest the receiver?

It doesn't make any difference.
 
W

whit3rd

Jan 1, 1970
0
When laying out a pcb/pwb, if a signal source needs to be ac coupled to a
receiver, is it best to:
1) put the cap nearest the source,
2) in the middle, or
3) nearest the receiver?

Depends on the source level and relative impedances on the
two sides of the capacitor. If the source could channel a lightning
strike, I'd want the capacitor to blow up right next to the
bulkhead feedthrough connector, where there's lots of grounded
metal.

And if the capacitor connects to a high-impedance receiver, the
receiver side is subject to spurious pickup and should be wired close.
A sufficiently large capacitor, of course, would swamp the pickup
current, BUT don't use one that big. Save your pennies.
 
L

legg

Jan 1, 1970
0
Hi
When laying out a pcb/pwb, if a signal source needs to be ac coupled to a
receiver, is it best to:
1) put the cap nearest the source,
2) in the middle, or
3) nearest the receiver?

Noise considerations would suggest that tracking to the first active
node of the reciever be minimized. As a coupling capacitor is intended
to look like a short at the frequencies communicated, it simply
increases the effective pick-up area of the tracking wherever it is
situated.

Singlre-fault abnormal implications would suggest that the capacitor
be closest to the node that would be most physically damaged by a
single fault to ground, or adjacent tracking. This could be either
node, but the transmission side usually has greater potential circuit
component loss under the abnormal fault, as it is configured to
deliver power. This is an issue of recoverability, repairability.

Series resistor tracking can have a larger affect on nodes subjected
to external noise sources than series coupling caps.

RL
 
G

Grumps

Jan 1, 1970
0
Andrew Holme said:
It doesn't make any difference.

Thanks.
I initially thought that.
But it got me thinking, and hence this question.
 
G

Grumps

Jan 1, 1970
0
Depends on the source level and relative impedances on the
two sides of the capacitor. If the source could channel a lightning
strike, I'd want the capacitor to blow up right next to the
bulkhead feedthrough connector, where there's lots of grounded
metal.

Thanks.
Not an issue in my situation though.
And if the capacitor connects to a high-impedance receiver, the
receiver side is subject to spurious pickup and should be wired close.
A sufficiently large capacitor, of course, would swamp the pickup
current, BUT don't use one that big. Save your pennies.

This is what I thought. I'll have to check the receiver input impedance.
 
G

Grumps

Jan 1, 1970
0
legg said:
Noise considerations would suggest that tracking to the first active
node of the reciever be minimized. As a coupling capacitor is intended
to look like a short at the frequencies communicated, it simply
increases the effective pick-up area of the tracking wherever it is
situated.

Singlre-fault abnormal implications would suggest that the capacitor
be closest to the node that would be most physically damaged by a
single fault to ground, or adjacent tracking. This could be either
node, but the transmission side usually has greater potential circuit
component loss under the abnormal fault, as it is configured to
deliver power. This is an issue of recoverability, repairability.

Series resistor tracking can have a larger affect on nodes subjected
to external noise sources than series coupling caps.

Thanks.
 
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