GS9068(2010) 查看數據表（PDF） - Gennum -> Semtech
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GS9068 Datasheet PDF : 11 Pages
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3.3 Output Return Loss Measurement
To perform a practical return loss measurement, it is necessary to force the GS9068
output to a DC high or low condition. The actual return loss will be based on the outputs
being static at VCC or VCC-1.6V. Under normal operating conditions, the outputs of the
GS9068 swing between VCC-0.4V and VCC-1.2V, hence the measured value of return
loss will not represent the actual operating return loss.
A simple method of calculating the values of actual operating return loss is to interpolate
the two return loss measurements. In this method, the values of return loss are estimated
at VCC-0.4V and VCC-1.2V based on the measurements at VCC and VCC-1.6V.
The two values of return loss (high and low) will typically differ by several decibels. If
the measured return loss is RH for logic high and RL for logic low, then the two values can
be interpolated as follows:
RIH = RH- (RH-RL)/4, and
RIL = RL+(RH-RL)/4
Where RIH is the interpolated logic high value and RIL is the interpolated logic low value.
For example: if RH = -18dB and RL = -14dB; the interpolated values are RIH = -17dB and
RIL = -15dB.
3.4 Output Amplitude Adjustment
The output amplitude of the GS9068 can be adjusted by changing the value of the RSET
resistor as shown in Figure 3-2 and Table 3-1 below. For an 800mVp-p output with a
nominal ±7% tolerance, a value of 750Ω is required. A ±1% SMT resistor should be used.
1100
1000
900
800
700
600
500
600
700
800
900
1000
RSET (W)
Figure 3-2: Output Amplitude Adjustment
The RSET resistor is part of the high-speed output circuit of the GS9068. The resistor
should be placed as close as possible to the RSET pin. In addition, the PCB capacitance
should be minimized at this node by removing the PCB ground plane beneath the RSET
resistor and the RSET pin.
GS9068 SD SDI Cable Driver
Data Sheet
22213 - 4
August 2010
7 of 11
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