GS2989 Dual-Slew-Rate, Dual-Output Cable Driver

with 3Gb/s Capability

Data Sheet

52133 - 4

December 2010

13 of 19

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 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.

4.7 Output Return Loss Measurement

The GS2989 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 both serial data differential outputs,

effectively stopping all outputs at the output common mode voltage level. Gennum

recommends using BALANCE mode when measuring ORL with 2.5V termination

voltage.

Table 4-5: Typical RSET Values

Output Swing (mV)

RSET (Ω)

1800*

332

800

750

500

1210

*NOTE: In order to generate output swings greater than 1040mV, VCC_TERM must be

connected to a 5V supply.