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-   -   3.3V Biasing (http://www.rdmprotocol.org/forums/showthread.php?t=56)

William CANO November 29th, 2006 02:48 AM

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

Nigel Worsley November 30th, 2006 09:12 AM

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.

William CANO November 30th, 2006 04:05 PM

Many Thanks Nigel,

Could you explain how to calculate ?

Kind regards

William

Nigel Worsley December 5th, 2006 08:17 AM

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.


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