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ADA4420-6ARQZ1 Arkusz danych(PDF) 10 Page - Analog Devices |
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ADA4420-6ARQZ1 Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 12 page ADA4420-6 Rev. 0 | Page 10 of 12 APPLICATIONS INFORMATION OVERVIEW With its high impedance inputs and high output drive, the ADA4420-6 is ideally suited to video reconstruction and anti- alias filtering applications. The high impedance inputs give designers flexibility with regard to how the input signals are terminated. Devices with DAC current source outputs that feed the ADA4420-6 can be loaded in whatever resistance provides the best performance, and devices with voltage outputs can be optimally terminated as well. The ADA4420-6 outputs can each drive up to two source-terminated, 75 Ω loads and, therefore, can directly drive the outputs from set-top boxes and DVDs without the need for a separate output buffer. DISABLE The ADA4420-6 includes a disable feature that can be used to save power when a particular device is not in use. When disabled, the ADA4420-6 typically draws only 7 μA from the supply. The disable feature is asserted by pulling the DIS pin low. Table 5 summarizes the operation of the disable feature. Table 5. Disable Function DIS Pin Connection Status VCC or floating Enabled GND Disabled INPUT AND OUTPUT COUPLING Inputs to the ADA4420-6 can be ac- or dc-coupled. For dc-coupled inputs, the signal must be completely contained within the input range of 0 V to 2.1 V. When using ac-coupled inputs, the lowest point of the signal is clamped to approximately 0 V. The ADA4420-6 outputs can be either ac- or dc-coupled. When driving single ac-coupled loads in standard 75 Ω video distribution systems, a minimum capacitance of 220 μF is recommended to avoid line and field droop. There are two ac coupling options when driving two loads from one output. One option simply uses the same value capacitor on the second load, while the other option uses a common coupling capacitor that is at least twice the value used for the single load (see Figure 20 and Figure 21). When driving two parallel 150 Ω loads (75 Ω effective load), the 3 dB bandwidth of the filters typically varies from that of the filters with a single 150 Ω load (see Figure 4). 75Ω 75Ω 75Ω CABLE 75Ω CABLE 220µF ADA4420-6 220µF 75Ω 75Ω Figure 20. Driving Two AC-Coupled Loads with Two Coupling Capacitors ADA4420-6 75Ω CABLE 75Ω CABLE 75Ω 75Ω 75Ω 75Ω 470µF Figure 21. Driving Two AC-Coupled Loads with One Common Coupling Capacitor PRINTED CIRCUIT BOARD (PCB) LAYOUT As with all high speed applications, attention to the PCB layout is of paramount importance. When designing with the ADA4420-6, adhere to standard high speed layout practices. A solid ground plane is recommended, and surface-mount, ceramic power supply decoupling capacitors should be placed as close as possible to the supply pins. Connect all of the ADA4420-6 GND pins to the ground plane with traces that are as short as possible. Controlled impedance traces of the shortest length possible should be used to connect to the signal I/O pins and should not pass over any voids in the ground plane. A 75 Ω impedance level is typically used in video applications. When driving transmission lines, include series termination resistors on the signal outputs of the ADA4420-6. When the ADA4420-6 receives its inputs from a device with current outputs, the required load resistor value for the output current is often different from the characteristic impedance of the signal traces. In this case, if the interconnections are short (<< 0.1 wavelength), the trace does not have to be terminated in its characteristic impedance. Traces of 75 Ω can be used in this instance, provided their lengths are an inch or two at most. This is easily achieved because the ADA4420-6 and the device feeding it are usually adjacent to each other, and connections can be made that are less than one inch in length. VIDEO ENCODER RECONSTRUCTION FILTER The ADA4420-6 is easily applied as a reconstruction filter at the DAC outputs of a video encoder. Figure 22 illustrates how to use the ADA4420-6 in this type of application following an ADV734x series video encoder, with a single-supply and ac-coupled outputs. |
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