GS2989

Final Data Sheet

Rev.6

GENDOC-052133

July 2015

13 of 20

Semtech

www.semtech.com

In order to determine the best starting value for Rset, the following formula should be

used:

Rset = 8*(Rtrm/VoutppSE)

Where VoutppSE is in Volts, and both resistances are in

Ω.

Rtrm is the value of the termination resistors, which should be equal to the

characteristic impedance of the cable, and is typically 75

Ω.

The cable must be short (

≤1m), and terminated at both ends for the formula to be valid.

Example: For a 75

Ω cable, Rtrm = 75Ω (at both ends), VoutppSE = 800mV

Rset = 8*(75/0.8) = 750

Ω

This formula is not valid for long, unterminated, or improperly terminated cables.

This formula should be considered as a starting point, and actual swing values may vary

based on layout. Also, for large output swings (>1040mV), smaller Rset values may be

required in order to achieve the desired output swing level at HD and 3G data rates.

1.75

1.5

1.25

1.0

0.75

0.5

0.25

2.0

0.250

0.500

0.750

1.0

1.25

1.5

RSET (kΩ)