Rule of thumb: 25% of all backplane connector pins have to be GND lines!

Thread Starter

AnalogDigitalDesigner

Joined Jan 22, 2018
121
I read this somewhere:

"Rule of thumb: 25% of all backplane connector pins have to be GND lines!"

Here is the file: ftp://ftp.dii.unisi.it/pub/users/vignoli/old/elettronicaii_cl/materiale_didattico/digital/per%20approfondire/bus_sys.pdf

Quote on last page!

Why is this ?
 

kubeek

Joined Sep 20, 2005
5,758
Basically each logic line should have its own ground pin for best signal integrity. Half of that seems like a good compromise, since not all lines will be toggling at the same time.
 

Thread Starter

AnalogDigitalDesigner

Joined Jan 22, 2018
121
Basically each logic line should have its own ground pin for best signal integrity. Half of that seems like a good compromise, since not all lines will be toggling at the same time.

Well you only repeated the quote using different words..... :p What is the real reason ? All ground pins are the same ground. I don't get why each signal needs its own ground ... ?
 

OBW0549

Joined Mar 2, 2015
3,566
The reason for multiple grounds is stated in the bullet point directly above the line you quoted: "Driver output current is 100 mA/line. Provide adequate and low inductance GND return path (simultaneous switching)!"

Multiple ground connections reduce both the resistance and the inductance of the ground path, which has to carry the return currents from all of the driven bus signals.
 

kubeek

Joined Sep 20, 2005
5,758
Well you only repeated the quote using different words..... :p What is the real reason ? All ground pins are the same ground. I don't get why each signal needs its own ground ... ?
Even for DC ground is not the same thing all over the board, due to resistance. High currents cause what is called ground bounce, where the ground potential in different parts of the system is at different voltage. At high frequencies this becomes even more significant, because due to inductance the impedance is even higher and the voltage difference between the ground potentials will be significant part of the useful signal, and can lead to things from wrong timing to completely wrong logic levels and resulting values.
Therefore you want as many ground pins as possible to minimize inductance (and to a lesser degree resistance) and to minimize influence of each signal on the other lines.
 

Thread Starter

AnalogDigitalDesigner

Joined Jan 22, 2018
121
The reason for multiple grounds is stated in the bullet point directly above the line you quoted: "Driver output current is 100 mA/line. Provide adequate and low inductance GND return path (simultaneous switching)!"

Multiple ground connections reduce both the resistance and the inductance of the ground path, which has to carry the return currents from all of the driven bus signals.

Not if I have a ground plane for all signals!
 

Thread Starter

AnalogDigitalDesigner

Joined Jan 22, 2018
121
Even for DC ground is not the same thing all over the board, due to resistance. High currents cause what is called ground bounce, where the ground potential in different parts of the system is at different voltage. At high frequencies this becomes even more significant, because due to inductance the impedance is even higher and the voltage difference between the ground potentials will be significant part of the useful signal, and can lead to things from wrong timing to completely wrong logic levels and resulting values.
Therefore you want as many ground pins as possible to minimize inductance (and to a lesser degree resistance) and to minimize influence of each signal on the other lines.


I have a big ground plane, so why would I need lots of GND pins son ?
 

kubeek

Joined Sep 20, 2005
5,758
I have a big ground plane, so why would I need lots of GND pins son ?
I am pretty sure I am not your son.
Nevertheless, if you want your ground plane to continue on the backplane board and act as a continous ground plane, then you need a lot of ground pins to connect the two, otherwise the two planes will be at different potentials and you will have issues with signal integrity. That is of course for signals with fast edges, if the signals are slow and low current you don´t need more than a few ground pins to get good results.
 

Thread Starter

AnalogDigitalDesigner

Joined Jan 22, 2018
121
I am pretty sure I am not your son.
Nevertheless, if you want your ground plane to continue on the backplane board and act as a continous ground plane, then you need a lot of ground pins to connect the two, otherwise the two planes will be at different potentials and you will have issues with signal integrity. That is of course for signals with fast edges, if the signals are slow and low current you don´t need more than a few ground pins to get good results.

Yep of course!
 
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