GS9068 datasheet(7/11 Pages) GENNUM

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GS9068 SD SDI Cable Driver

Data Sheet

22213 - 4

August 2010

7 of 11

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.

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.

1100

1000

900

800

700

800

900

1000

SET (W)

600

500

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