[SI-LIST] Re: Rise time calculation
- From: "Ravikumar Chirugudu" <ravikumar.chirugudu@xxxxxxxxx>
- To: SI List <si-list@xxxxxxxxxxxxx>
- Date: Tue, 31 Jul 2001 10:53:23 +0530
Hello,
It seems there is a fallacy in the argument. Source termination is decided
based on following :
1. to avoid transmission line reflections by proper termination.
2. the value is selected based on the drive capability of the driver. If the
driver has 20 Ohm
output impedance, we may have to place 30 Ohm for proper termination when using
50 Ohm trace to take
care of (1). But if the trace length is so large or the drive strength is so
poor inspite of good
rise time at the output of the driver, which normally is the case, the series
resistor will not be
30 Ohm. The value may have to be less than 20 Ohm.
What I feel is, this should be treated as a low pass RC combination rather than
the equation given
below. The equation below does not include the series termination value; the
assumption of having
Zo in the equation leads to the fact that the series termination is the
difference between the
characteristic impedance and the source impedance. But that is not the case in
practical. The
series termination will be different. I feel that treat R as the source
termination resistance
and C as the total capacitive load (trace + pin + load) and apply 2.2RC for
getting the rise time
(assuming infinite current source). Even for a slow logic like TTL, the peak
currents are as high
as 600 mA. Hence it can be approximated that the driver is a infinite AC
current source.
Please comment.
Regards,
Ravi
subramanya C K wrote:
> Good Morning all,
> here are my two cents to the topic.
>
> It's been observed by practice (and also proved mathematically) that a
> series termination adds up to propagation delay and also slows down the fast
> rising edges.
> Some amount of mathematics can be used to prove that propagation delay
> introduced by series termination is double the propagation delay introduced
> by the parallel termination.
>
> Following equations might help you on this front;
>
> If Zo is the intrinsic impedance of the trace (transmission line), Ct is the
> Thevenin equivalent load capacitance and
> Tpd is the propagation delay of the trace,
>
> then:
> Tpd = 0.70 X Zo X Ct, for a series terminated line and
> Tpd = 0.35 X Zo X Ct, for a parallel terminated line.
>
> Some more mathematical calculations provide the following results;
>
> If Tri is the input rise time, (ie., Rise time of the source) and Tro is the
> output rise time(at the output end of termination resistor),
>
> then:
>
> Tro = (Zo X Ct) X ln [ {(Zo X Ct)/(0.18 X Tri)} X
> [{e^{(1.2 X Tri)/(Zo X
> Ct)}-1}],
> for Series terminated line.
> and
> Tro = {(Zo X Ct)/2} X ln [ {(Zo X Ct)/(0.36 X Tri)} X
> [{e^{(2.4 X
> Tri)/(Zo X Ct)}-1}]
> for parallel terminated line.
>
> Hope this will help.
>
> Best Regards
> Subramanya C K
> Wipro Technologies, Bangalore- India
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Jinto N.Jose
> Sent: Monday, July 30, 2001 3:12 PM
> To: SI List
> Subject: [SI-LIST] Rise time calculation
>
> Hi all,
> How should I calculate the rise time at the receiver of a series
> terminated
> signal ? What all parameters I need to know for doing this calculation ?
>
> Driver Trace of known
> |\ Rs impedance & length |\
> | \__/\/\/\__|------------------|_| \
> | / |------------------| | /
> |/ |/
> Receiver
>
> I want to calculate the rise time at the receiver input.
>
> Thanks
> Jinto
>
> -- Attached file included as plaintext by Listar --
> -- File: Wipro_Disclaimer.txt
>
> The Information contained and transmitted by this E-MAIL is proprietary to
> Wipro Limited and is intended for use only by the individual or entity to
> which
> it is addressed, and may contain information that is privileged,
> confidential or
> exempt from disclosure under applicable law. If this is a forwarded message,
> the content of this E-MAIL may not have been sent with the authority of the
> Company. If you are not the intended recipient, an agent of the intended
> recipient or a person responsible for delivering the information to the
> named
> recipient, you are notified that any use, distribution, transmission,
> printing,
> copying or dissemination of this information in any way or in any manner is
> strictly prohibited. If you have received this communication in error,
> please
> delete this mail & notify us immediately at mailadmin@xxxxxxxxx
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
> -- Attached file included as plaintext by Listar --
> -- File: Wipro_Disclaimer.txt
>
> The Information contained and transmitted by this E-MAIL is proprietary to
> Wipro Limited and is intended for use only by the individual or entity to
> which
> it is addressed, and may contain information that is privileged, confidential
> or
> exempt from disclosure under applicable law. If this is a forwarded message,
> the content of this E-MAIL may not have been sent with the authority of the
> Company. If you are not the intended recipient, an agent of the intended
> recipient or a person responsible for delivering the information to the named
> recipient, you are notified that any use, distribution, transmission,
> printing,
> copying or dissemination of this information in any way or in any manner is
> strictly prohibited. If you have received this communication in error, please
> delete this mail & notify us immediately at mailadmin@xxxxxxxxx
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
-- Attached file included as plaintext by Listar --
-- File: Wipro_Disclaimer.txt
The Information contained and transmitted by this E-MAIL is proprietary to
Wipro Limited and is intended for use only by the individual or entity to
which
it is addressed, and may contain information that is privileged, confidential
or
exempt from disclosure under applicable law. If this is a forwarded message,
the content of this E-MAIL may not have been sent with the authority of the
Company. If you are not the intended recipient, an agent of the intended
recipient or a person responsible for delivering the information to the named
recipient, you are notified that any use, distribution, transmission,
printing,
copying or dissemination of this information in any way or in any manner is
strictly prohibited. If you have received this communication in error, please
delete this mail & notify us immediately at mailadmin@xxxxxxxxx
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts: