Re: [SI-LIST] : Transmission line termination

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From: Ashok Babu K (k.ashokbabu@gdatech.co.in)
Date: Sat Apr 07 2001 - 01:57:25 PDT


Both the terminations are the same as far as the AC signals are concerned.
No reflections can be expected at the receiver inputs. But the difference
arises when DC conditions are concerned. In the latter case the receiver is
terminated with 50ohm resistance biased at Vtt. This causes the difference
in the common mode DC current flowing thru the termination resistance.
Usually the value of Vtt is chosen in such a way to reduce the common mode
DC current.
For example, in PECL the common mode DC voltage at
the PECL output lines is about Vcc-1.3V. The PECL outputs drive the 50ohm
termination resistance at Vcc-2V ( i.e Vtt = Vcc-2V). So the common mode DC
current is 14mA. If it is not terminated with that bias voltage, and instead
grounded, the
common mode current will be more which would eventually cause more power
dissipation.

Regards,
Ashok.

----- Original Message -----
From: <sunil-chandra.kasanyal@st.com>
To: <si-list@silab.eng.sun.com>
Sent: Friday, April 06, 2001 1:27 PM
Subject: [SI-LIST] : Transmission line termination

>
> Hello all,
> What is the difference between these two terminations?Why?
> 1)
> -----------------------
> DRIVER->-----| TRANSMISSION LINE |-------------RECIEVER
> ----------------------- \
> Zo=50ohms /Rtt=50ohms
> \
> /
> ---GND
> 2)
>
> -----------------------
> DRIVER->-----| TRANSMISSION LINE |-------------RECIEVER
> ----------------------- \
> Zo=50ohms /Rtt=50ohms
> \
> /
> ------TERMINATION VOLTAGE=Vtt
>
>
> If signals of 5V come from drivers,no reflection will take place at the
> reciever ends.Since in above two cases,way of termination is
> different,what exactly the different uses of these two terminations.
>
> Regards
> Sunil
>
>
>
>
>
>
>
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