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coolman1977
Joined: 26 Nov 2007
Posts: 4
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 30 Jul 2008 6:53 Impedence controlled signal |
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Hi all,
I had a question related to controlled impedence signals i.e
Depending on what parameter is the signals impedance values are standardized like all the USB differential pairs are routed using 90 Ohms impedance, DDR signals with 50 Ohms and differential signals with 100 E.
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House_Cat
Joined: 21 Feb 2002
Posts: 1356
Helped: 275
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| 30 Jul 2008 20:31 Re: Impedence controlled signal |
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No, there is no single document that specifies what impedance all signals have to be. The ones you quote do come from the individual specifications for USB, DDR, etc. But at the same time, other signals are less tightly controlled in those same applications. For example, single ended USB signals can be between 45 and 80 ohms - so what you would use for your specific PCB design would depend on the chip manufacturer.
You have to look at the individual specifications for the type of device with which you are working, and also the chip manufacturer's data for the chipsets you are using.
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FvM
Joined: 22 Jan 2008
Posts: 2679
Helped: 438
Location: Bochum, Germany
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| 30 Jul 2008 23:28 Impedence controlled signal |
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| As youd mentioned, USB single ended impedance is only roughly specified. That's cause it's rather uncritical (only used to transmit SE0), but it doesn't depend on the chip manufacturer, I thnk.
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House_Cat
Joined: 21 Feb 2002
Posts: 1356
Helped: 275
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| 31 Jul 2008 0:49 Re: Impedence controlled signal |
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| The recommended termination is up to the chip manufacturer within the boundaries of the standard. You would want to match your track impedance to the termination impedance. Therefore, you need to look at both the standards AND the manufacturer's datasheet.
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