Datasheet AD8010 (Analog Devices) - 9

AD8010 APPLICATIONS
The bypassing scheme that is used for the AD8010 requires
Video Distribution Amplifier
special attention. It was found that the conventional technique The AD8010 is optimized for the specific function of providing of bypassing each power pin individually to ground can have an excellent video performance when driving multiple video loads adverse effect on the differential phase error of the circuit. The in parallel. Significant power is saved and heat sinking is greatly cause of this is attributed to the fact that there is an internal simplified because of the ability of the AD8010 to obtain this compensation capacitor in the AD8010 that is referenced to the performance when running on a ± 5 V supply. However, due to negative supply. the high currents that flow when driving many parallel video The recommended technique is to connect parallel bypass loads, special layout and bypassing techniques are required to capacitors from the positive supply to the negative supply and assure optimal performance. then to bypass the negative supply to ground. For high fre- When designing a video distribution amplifier with the AD8010, it quency bypassing, 0.1 µF ceramic capacitors are recommended. is very important to keep in mind where the high (ac) currents These should be placed within a few millimeters of the power will flow. These paths include the power supply pins of the chip pins and should preferably be chip type capacitors. along with the bypass capacitors and the return path for these The high currents that can potentially flow through the power capacitors, the output circuits and the return path of the output supply pins require large bypassing capacitors. These should be current from the loads. low inductance tantalum types and at least 47 µF. The ground In general, any loops that are formed by any of the above paths side of the capacitor that bypasses the negative supply should be should be made as small as possible. Large loops are both gen- brought to a single point ground that is the common for the erators and receivers of magnetic fields and can cause undesired returns of the outputs. coupling of signals that lowers the performance of the amplifier. Figure 30 shows a circuit for making an N-channel video distri- Effects that have not been seen before in other op amp circuits bution amplifier. As a practical matter, the AD8010 can readily might arise because of the high currents. Most op amp circuits drive eight standard 150 Ω video loads. When driving up to 12 output, at most, tens of milliamps and do not require extremely video loads, there is minimal degradation in video performance. tight video specifications, while a video distribution amplifier can output hundreds of milliamps and require extremely low Another important consideration when driving multiple cables differential gain and phase errors. is the high frequency isolation between the outputs of the cables. Due to its low output impedance, the AD8010 achieves better than 46 dB of output-to-output isolation at 5 MHz driv- ing back terminated 75 Ω cables.
+5V FB 499 499 75 AD8010 150 75 R V L1 IN C1 75 75 C2 75 RL2 FB –5V 75 75 RLN
Figure 30. An N-Channel Video Distribution Amplifier Using An AD8010. NOTE: Please see Figure 29 for Recommended Bypassing Technique. REV. B –9–