E1.20 RDM (Remote Device Management) Protocol Forums  

Go Back   E1.20 RDM (Remote Device Management) Protocol Forums > RDM Developer Forums > RDM Physical Layer/Hardware Discussion

RDM Physical Layer/Hardware Discussion Discussion and questions relating to the physical or hardware layer of RDM.

Reply
 
Thread Tools Search this Thread Display Modes
Old November 29th, 2006   #1
William CANO
Junior Member
 
Join Date: Nov 2006
Posts: 2
Unhappy 3.3V Biasing

Hi,

I plan t bias in 3.3V ( in spite of 5V).

I don't understand how to calculate the right valus for the controler resistors.

Do you have any idea ?

William
William CANO is offline   Reply With Quote
Old November 30th, 2006   #2
Nigel Worsley
Junior Member
 
Join Date: Jun 2006
Location: London
Posts: 13
Default

Change Rt1 to 150R and R1, R2 to 390R.

This gives almost exactly the same bias voltage, and is reasonably close to the 120R impedance requirement ( 126R ). Using difficult to obtain ( in small quantities ) values could provide a slightly closer match, but there is little point as the cable impedance will probably vary by far more than this anyway.
Nigel Worsley is offline   Reply With Quote
Old November 30th, 2006   #3
William CANO
Junior Member
 
Join Date: Nov 2006
Posts: 2
Default

Many Thanks Nigel,

Could you explain how to calculate ?

Kind regards

William
William CANO is offline   Reply With Quote
Old December 5th, 2006   #4
Nigel Worsley
Junior Member
 
Join Date: Jun 2006
Location: London
Posts: 13
Default

Th equations are:

Vbias = Vcc * 1 / ( 1 / Rt1 + 1 / Rt2 ) / ( 1 / ( 1 / Rt1 + 1 / Rt2 ) + R1 + R2 )

Rterm = 1 / ( 1 / Rt1 + 1 / ( R1 + R2 ) )

The specification requires Vbias to be greater than 245mV and Rterm to be 120R +/- 10%

Calculation isn't all that simple, successive approximation with the aid of a apreadsheet is probably the easiest way.

If you settle on 250mV bias voltage, then the values work out as:

R1 and R2 = 396R
Rt1 = 141.4R
Vbias = 249.98mV
Rterm = 119.98

Neither of these are standard resistor values, the nearest are 397R and 142R. However, these are part of the E192 series, which means that they are often impossible to obtain in small ( less than ten thousand ) quantities. The nearest readily available values are 390R and 150R, the values I originally suggested.

This gives 259.8 mV and 125.8R which is close enough. If you want a better impedance match, then reduce R1 and R2 to 300R, which will give an exact match but increases the bias voltage to 330mV. This isn't likely to be high enough to cause a problem, but neither will the 125.8R impedance.

Note that the power dissipation in Rt1 is likely to be about 170mW, this will be too much for most surface mount resistors.
Nigel Worsley is offline   Reply With Quote
Reply

Bookmarks

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Line biasing Flaws - need solutions berntd RDM Physical Layer/Hardware Discussion 7 April 9th, 2009 06:32 PM
Line biasing Andy Macdonald RDM Physical Layer/Hardware Discussion 1 October 26th, 2006 12:02 PM


All times are GMT -6. The time now is 01:48 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2024, vBulletin Solutions, Inc.