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AM7996 数据表(PDF) 9 Page - Advanced Micro Devices |
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AM7996 数据表(HTML) 9 Page - Advanced Micro Devices |
9 / 18 page AMD 9 Am7996 LAYOUT CONSIDERATIONS To protect the transceiver from the environment and to achieve optimum performance, the Am7996 is designed to be used with two sets of external components: the transmitter circuit consisting of components D1, D2, D3, R7, R8, and C3, and the receiver circuit consisting of components R5, R6, CL, and CC, (CL is a parasitic ca- pacitance rather than a discrete component). These two circuits are shown in both Figure 2 and in Figure 3 re- spectively. The resistor tolerances for these circuits are specified as 1% for temperature stability. The only layout restriction for the transmitter circuit is that the longest current path from the TXT pin (Pin 12) to the coaxial cable’s center conductor must be no longer than 4 inches. The layout of the receiver circuit, however, is critical. To minimize parasitic capacitance that can degrade the re- ceived signal, the external receiver circuit should be iso- lated from power and ground planes. There must be no power or ground plane under the area of the PC board that includes pins 15 through 20, R5, R6, and the con- nector for the coaxial cable. If a power or ground plane extends under this area, the receiver will not function properly due to excessive crosstalk and under- or over- compensation of the R5, R6 attenuator. Also, the RXT pin (Pin 16) should be as close to the coaxial cable connector as possible. Since there are no severe layout restrictions on the transmitter circuit, the layout can be simplified by omit- ting power and ground planes from the whole area on the right side of the Am7996 as shown in Figure 4-1. 07506E-7 20 RXT Am7996 TXT 11 R5, R6, R4 C1, C2 D1, D2, D3 R7, R8 C3 Area with no power or ground plane Coax Connector Figure 4-1. If the above layout rules are followed, the parasitic capacitance in parallel with R6 will be about 6 pF. This parasitic capacitance is shown in the schematics as CL (Note that CL is a parasitic capacitance. Do not add a discrete capacitor in parallel with R6). The capacitor labeled CC in the schematics is the total capacitance in parallel with R5 including parasitic capacitance. The parasitic component of CC will be about 1 pF. For opti- mum performance, the ratio of CL to CC should be the same as the ration of R5 to R6, which is 3 to 1. This means that an additional 1 pF of capacitance must be added in parallel with R5. This additional capacitance can easily be added by building a parallel-plate capacitor for PC traces right un- der resistor R5. This capacitor can consist of a 0.200 in. by 0.200 in. square of conductor on each side of the board as shown in Figure 4-2 (These dimensions as- sume that the PC board is made from 0.060 in. thick G-10 material). The top plate of the capacitor should be connected to one lead of R5, and the bottom plate should be connected to the other lead. Figure 4-3 shows an example of this suggested layout for a four layer printed circuit board. Note that the component labeling used in Figure 4-3 is not intended to correspond with the component labeling used in Figure 2 and Figure 3. Circuit Side Component Side 0.200 in x 0.200 in two planes R5 07506E-8 Figure 4-2. |
类似零件编号 - AM7996 |
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类似说明 - AM7996 |
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