AD9742_several questions_DCOM and ACOM

Question Why the shield of CLOCK is connected to DCOM and the clock is connected to ACOM through a pull-down resistor. Is there something wrong with it? I think they all should be DCOM. Answer Yes you are right. It should be DCOM for these grounds.

AD9742_several questions_differential resistor

Question The question is about the differential resistor in Figure 31. Is the ratio of Rdiff : Rload should be same with the ratio of transformer's impedance. The circuit will work normally if we do not add Rdiff but will make the VSWR higher. Is that right? Answer The resistance presented in the schematic you attached, will be Rload/2 if RDIFF and Rload are the same values. The idea of Rdiff is to present an impedance close to the part that will assure good termination and another one far away but where you do not have to be as careful for line impedance matching. Removing RDIFF is not recommended for this reason without it you will have to be careful of the impedance you are presenting at the pin level and it is risky with a transformer (not the most stable in term of impedance over frequency).

AD9742/AD9744 Exposed Pad

Question Reading through the AD9742's datasheet I cannot find what the exposed pad should be tied to? Answer The Exposed Thermal Pad must be soldered to copper pour on top surface of PCB for mechanical stability and must be electrically tied to low impedance GND plane for low noise performance.

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AD9742 Production
The AD97421 is a 12-bit resolution, wideband, third generation member of the TxDAC series of high performance, low power CMOS digital-to-analog converters...
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AD9742 on Analog.com