GS6081 Datasheet, PDF (12/19 Pages) Semtech Corporation – Input signal trace equalizationnull

Table 4-4: Signal Presence Indicator
Pre-Driver Output
Signal present
No signal present
SP Pin
0
1
4.6 Output Amplitude (RSET)
The output amplitude of the GS6081 can be adjusted by changing the value of the R_SET
resistor as shown in Figure 4-1. For an 800mV_pp output with a nominal ±7% tolerance, a
value of 750Ω is required. A ±1% SMT resistor should be used.
The R_SET resistor is part of an internal DC feedback loop in the GS6081. The resistor
should be placed as close as possible to the RSET pin, and connected directly to the V_CC
plane (traces/wires may cause instability).
Note: Care should be taken when considering layout of the R_SET resistor. Please refer to
Section 5.1 for more details.
1.25
1.0
0.75
0.5
0.25
0.500
0.750
1.0
1.25
1.5
RSET (kΩ)
Figure 4-1: V_OUT vs. R_SET
In order to determine the best starting value for R_SET, 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_pp
GS6081
Final Data Sheet
PDS-060046
Rev.4
July 2014
www.semtech.com
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