33 ohms series termination

[ashishbondia]

Why do we use 33 ohm termination on digital lines and where should be they placed (near to transmitter or receiver or in the middle, if the line is bidirectional)

[Joris]

Originally Posted by ashishbondia

Why do we use 33 ohm termination on digital lines and where should be they placed (near to transmitter or receiver or in the middle, if the line is bidirectional)

Hi,

I found something like that in my CD-ROM cd-transport. I just removed it, it improved sound quality. I think it is used to limit the possible current when the ouput of the digital signal is shorted.

Kind regards,
Joris

[jean-paul]

It is to filter RF out of the signals in combination with the input capacitance of the following chip. Also to limit currents.

[janneman]

Originally Posted by Joris

Hi,

I found something like that in my CD-ROM cd-transport. I just removed it, it improved sound quality. I think it is used to limit the possible current when the ouput of the digital signal is shorted.

Kind regards,
Joris

If it is termination, it is between signal and ground, like a load. It does not limit current (it takes current!). The purpose is to terminate the line to improve the signal shape (square wave). Without it you get a lot of ringing and phase shift.

jd

[jean-paul]

It is all in the word "series" janneman.

[janneman]

Originally Posted by jean-paul

It is all in the word "series" janneman.

I didn't see any 'series' word, J-P. Anyway, the OP talked about 'termination' and that is normally from line to ground. You can even have active termination to improve the signal and suppress excessive ringing.
It's often used on high speed digital lines where signal integrity is important.

Edit: http://en.wikipedia.org/wiki/Electrical_termination

Edit 2: for bidirectional lines usually there is termination on both ends.

Edit 3: Ahh yes, the series word is in the thread title. OK.

jd

[aparatusonitus]

Originally Posted by ashishbondia

Why do we use 33 ohm termination on digital lines and where should be they placed (near to transmitter or receiver or in the middle, if the line is bidirectional)

If it's transmitter -> receiver case, resistor is close to transmitter, cap is close to receiver.

For bidirectional line...well I'm not that smart

[janneman]

Originally Posted by aparatusonitus

If it's transmitter -> receiver case, resistor is close to transmitter, cap is close to receiver.

For bidirectional line...well I'm not that smart

No, termination is always at the receiver side. The waveform integrity at the transmitter end is controlled by the transmitter, it's at the receiver where you need to control ringing and reflection.
And what's that cap? There's no cap in a termination.

BTW Do we agree we talk about termination as in signal to ground termination, NOT any series impedance? If it is series impedance, it's not termination in the normal sense, maybe more current limiting.

jd

[janneman]

Originally Posted by janneman

No, termination is always at the receiver side. The waveform integrity at the transmitter end is controlled by the transmitter, it's at the receiver where you need to control ringing and reflection.
And what's that cap? There's no cap in a termination.

BTW Do we agree we talk about termination as in signal to ground termination, NOT any series impedance? If it is series impedance, it's not termination in the normal sense, maybe more current limiting.

jd

Just thought about something. If it is a series resistor at the source (or receiver) and a cap to ground, that would of course slow down the rise- and fall times and decrease ringing. That would be suboptimal imho; the slower rise/fiall times make the switching point less well defined and will increase signal jitter.

jd

[aparatusonitus]

Don't forget to add wire lenght, so you will have RLC LP filter at the end.