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rogger123
Joined: 09 Apr 2003
Posts: 114
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 15 Oct 2004 16:26 bus driver Q |
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| consider we have a driver that drives a long signal & connects to an input device. At the input device there is either overshoot, undershoot or signal threshold violations, what are the various things that can be done to correct this problem
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papyaki
Joined: 13 Apr 2002
Posts: 564
Helped: 19
Location: A small village somewhere in Gaul
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| 15 Oct 2004 16:53 Re: bus driver Q |
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Hi,
To avoid this kind of problem, you can use one of these solution :
1) Add a serial resistance at the driver's output.
2) Add two resistances at the device input, one between signal and Vcc, the second one between signal and Ground.
3) Add one resistance equal to the caracteristic impedance of the wire at the device input, between signal and Ground
Look page 18 of this pdf for details :
h**p://www.national.com/an/AN/AN-991.pdf
* = t
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cesare
Joined: 21 Feb 2002
Posts: 177
Helped: 7
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| 16 Oct 2004 8:35 Re: bus driver Q |
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| Usually the simplest thing to do is to add a series resistor at the output of the driver (at the beginning of the line). The sum of the driver's output resistance and the series resistor must match the line impedance.
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rogger123
Joined: 09 Apr 2003
Posts: 114
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| 16 Oct 2004 15:12 Re: bus driver Q |
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well it seems using the resistors is the best solution.
but could any one explain to me how this solve the problem???
how did the addition of the resistor eleiminate the problem?
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papyaki
Joined: 13 Apr 2002
Posts: 564
Helped: 19
Location: A small village somewhere in Gaul
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| 16 Oct 2004 16:48 Re: bus driver Q |
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Hi,
All is very well explained in AN991. Just download and read it.
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cesare
Joined: 21 Feb 2002
Posts: 177
Helped: 7
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| 19 Oct 2004 15:51 Re: bus driver Q |
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It works because usually the driver output impedance is lower than line impedance.
If the series resistor is not present the line is mismatched both at the drive end and at the load end. In such a situation you have reflections at both ends of the line and this results in overshoots and undershoots at both ends.
If the series resistor is close to the driver, the short trace between the driver and the resistor can be neglected and the driver has an output resistance equal to the line impedance. In this way the line is matched at the source but not at the load.
The effect of this is that you have only one reflection at the load end of the line.
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