4.2.1 Output Amplitude (RSET)
The output amplitude of the GS2978 can be adjusted by changing the value of the R
SET
resistor as shown in Table 4-1. For an 800mV output with a nominal ±7% tolerance, a
p-p
value of 750Ω is required. A ±1% SMT resistor should be used.
The R resistor is part of the high speed output circuit of the GS2978. The resistor
SET
should be placed as close as possible to the R pin. In addition, the PCB capacitance
SET
should be minimized at this node by removing the PCB groundplane beneath the R
SET
resistor and the R pin.
SET
Table 4-1: R
vs. V
OD
SET
RSET R (Ω)
Output Swing (mVp-p)
995
824
750
680
573
608
734
800
884
1040
NOTE: For reliable operation of the GS2978 over the full temperature range, do not use an
SET value below 573Ω.
R
4.2.2 Output Disable
The serial output disable (DISABLE), disables power to the current mode serial digital
output driver. When asserted LOW, the SDO/SDO output driver is powered off.
SDO/SDO will float to V through the pull-up resistor.
CC
NOTE: If the DISABLE pin is left as a No Connect (NC), the SDO/SDO outputs are still
active.
4.3 Output Return Loss Measurement
To perform a practical return loss measurement, it is necessary to force the GS2978
output to a DC high or low condition. The actual measured return loss will be based on
the outputs being static at V or V -1.6V. Under normal operating conditions the
CC
CC
outputs of the device swing between V -0.4V and V -1.2V.
CC
CC
GS2978
9 of 15
HD-LINX® III Multi-Rate Dual Slew-Rate Cable Driver
Data Sheet
37186 - 7
March 2009
GENNUM [ GENNUM CORPORATION ]
