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AD8010 Datenblatt(PDF) 9 Page - Analog Devices |
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AD8010 Datenblatt(HTML) 9 Page - Analog Devices |
9 / 12 page AD8010 –9– REV. A APPLICATIONS Video Distribution Amplifier The AD8010 is optimized for the specific function of providing excellent video performance when driving multiple video loads in parallel. Significant power is saved and heat sinking is greatly simplified because of the ability of the AD8010 to obtain this performance when running on a ±5 V supply. However, due to the high currents that flow when driving many parallel video loads, special layout and bypassing techniques are required to assure optimal performance. When designing a video distribution amplifier with the AD8010, it is very important to keep in mind where the high (ac) currents will flow. These paths include the power supply pins of the chip along with the bypass capacitors and the return path for these capacitors, the output circuits and the return path of the output current from the loads. In general, any loops that are formed by any of the above paths should be made as small as possible. Large loops are both gen- erators and receivers of magnetic fields and can cause undesired coupling of signals that lowers the performance of the amplifier. Effects that have not been seen before in other op amp circuits might arise because of the high currents. Most op amp circuits output, at most, tens of milliamps and do not require extremely tight video specifications, while a video distribution amplifier can output hundreds of milliamps and require extremely low differential gain and phase errors. The bypassing scheme that is used for the AD8010 requires special attention. It was found that the conventional technique of bypassing each power pin individually to ground can have an adverse effect on the differential phase error of the circuit. The cause of this is attributed to the fact that there is an internal compensation capacitor in the AD8010 that is referenced to the negative supply. The recommended technique is to connect parallel bypass capacitors from the positive supply to the negative supply and then to bypass the negative supply to ground. For high fre- quency bypassing, 0.1 µF ceramic capacitors are recommended. These should be placed within a few millimeters of the power pins and should preferably be chip type capacitors. The high currents that can potentially flow through the power supply pins require large bypassing capacitors. These should be low inductance tantalum types and at least 47 µF. The ground side of the capacitor that bypasses the negative supply should be brought to a single point ground that is the common for the returns of the outputs. Figure 30 shows a circuit for making an N-channel video distri- bution amplifier. As a practical matter, the AD8010 can readily drive eight standard 150 Ω video loads. When driving up to 12 video loads, there is minimal degradation in video performance. Another important consideration when driving multiple cables 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. Figure 30. An N-Channel Video Distribution Amplifier Using An AD8010. NOTE: Please see Figure 29 for Recommended Bypassing Technique. AD8010 VIN 150 75 RL1 499 499 +5V –5V 75 75 75 75 75 RL2 75 RLN FB FB C1 C2 |
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Ähnliche Beschreibung - AD8010 |
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