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GS9068 Datasheet(PDF) 5 Page - Gennum Corporation |
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GS9068 Datasheet(HTML) 5 Page - Gennum Corporation |
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5 / 8 page  GENNUM CORPORATION 22213 - 2 5 of 8 3. DETAILED DESCRIPTION 3.1 SERIAL DIGITAL INPUT SDI/SDI are high impedance differential inputs. Several conditions must be observed when interfacing to these inputs: 1. The differential input signal amplitude must be between 300 and 2000mVpp. 2. For DC coupling to the device, the common mode voltage must be between 1.6+ ∆V SDI(DIFF) and VCC- ∆V SDI(DIFF). 3. For input trace lengths longer than approximately 1cm, the inputs should be terminated as shown in the Typical Application Circuit. The GS9068 inputs are self-biased, allowing for simple AC coupling to the device. For serial digital video, a minimum capacitor value of 4.7µF should be used to allow coupling of pathological test signals. A tantalum capacitor is recommended. 3.2 SERIAL DIGITAL OUTPUT The GS9068 outputs are current mode and will drive 800mV into a 75 Ω load. These outputs are protected from accidental static damage with internal static protection diodes. The SMPTE 259M standard requires that the output of a cable driver have a source impedance of 75 Ω and a return loss of at least 15dB between 5MHz and 540MHz. In order for an SDI output circuit using the GS9068 to meet this specification, the output circuit shown in the Typical Application Circuit is recommended. The value of LCOMP will vary depending on the PCB layout, with a typical value of 5.6nH. A 4.7µF capacitor is used for AC coupling the output of the GS9068. This value is chosen to ensure that pathological signals can be coupled without a significant DC component occurring. See Section 4, Application Reference Design, for more details. When measuring return loss at the GS9068 output, it is necessary to take the measurement for both a logic high and a logic low output condition. This is because the output protection diodes act as a varactor (voltage controlled capacitor) as shown in Figure 1. Consequently, the output capacitance of the GS9068 is dependent on the logic state of the output. Fig. 1 Static Protection Diodes 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, so 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 way, 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, then the interpolated values are RIH = -17dB and RIL = -15dB. SDO SDO |
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Similar Description - GS9068 |
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