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 (V), and both resistances are in ohms (Ω).
Rtrm is the value of the termination resistors, which should be equal to the
characteristic impedance of the cable, and is typically 75Ω.
The cable must be short (≤1m), and terminated at both ends for the formula to be valid.
Example: For a 75Ω cable, Rtrm = 75Ω (at both ends), VoutppSE = 800mV
Rset = 8*(75/0.8) = 750Ω
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.
Table 4-5: Typical RSET Values
Output Swing (mV)
RSET (Ω)
1800*
800
332
750
500
1210
*Note: In order to generate output swings greater than 1040mV, VCC_TERM must be connected to a 5V
supply.
4.7 Output Return Loss Measurement
The GS2988 has a feature which allows users to measure ORL reliably while the device is
still powered. The device can be put into a Balance mode which prevents the outputs
from toggling while the device is powered on, allowing the ORL to be measured while
the device is still powered.
When EQ_EN is LOW, while SD/HD is HIGH, the device goes into Balance mode. This
mode is used during ORL measurement, disabling the AC signal path of the device
without powering it down. When in Balance mode, the device produces equal
pull-down currents in both differential shoulders of the serial data differential output,
effectively stopping the output at the output common mode voltage level. Semtech
recommends using Balance mode when measuring ORL with 2.5V termination voltage.
GS2988
Final Data Sheet
13 of 18
Semtech
www.semtech.com
Rev.6
GENDOC-052131 July 2015
SEMTECH [ SEMTECH CORPORATION ]
