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AD668AQ Datasheet(PDF) 11 Page - Analog Devices |
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AD668AQ Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD668 REV. A –11– reference input. While the two DACs are similar in many ways, the optimal decoupling schemes differ between the two parts and care should be used to insure that each is implemented appropriately. Figure 13. AD568 Driving the AD668 CONSTRUCTION GUIDELINES HIGH FREQUENCY PRINTED CIRCUIT BOARD SUGGESTIONS In systems seeking to simultaneously achieve high speed and high accuracy, the implementation and construction of the cir- cuit is often as important as the circuit’s design. Proper RF techniques must be used in device selection, placement and routing, and supply bypassing and grounding. In many areas, the performance of the AD668 may exceed the measurement ca- pabilities of common lab instruments, making performance evaluation particularly difficult. The AD668 has been config- ured to be relatively easy to use in spite of these problems, and realization of the performance indicated in this datasheet should not be difficult if proper care is taken. Figure 14 provides an il- lustration of the printed circuit board layout used for much of the AD668’s characterization. The board represents an imple- mentation of the circuit shown in Figure 23, with the AD586 used to drive the reference channel (as in Figure 11). Component Side Foil Side Figure 14. PC Board Layout THE USE OF GROUND AND POWER PLANES If properly implemented, ground planes can perform a myriad of functions on high speed circuit boards: bypassing, shielding, current transport, etc. In mixed signal design, the analog and digital portions of the board should be distinct from one an- other, with the analog ground plane confined to areas covering analog signal traces and the digital ground plane confined to areas covering digital interconnect. The two ground planes should be connected by paths 1/4 inch to 1/2 inch wide on both sides of the DAC, as shown in Figure 14. Care should be taken to insure that the ground plane is uninterrupted over crucial sig- nal paths. On the digital side, this includes the digital input lines running to the DAC, as well as any clock signals. On the analog side, this includes the analog input signal, the DAC output sig- nal, and the supply feeders. The use of wide runs or planes in the routing of the power supplies is also recommended. This serves the dual function of providing a low series impedance power supply to the part as well as providing some “free” ca- pacitive decoupling to the appropriate ground plane. USING THE RIGHT BYPASS CAPACITORS The capacitors used to bypass the power supplies are probably the most important external components in any high speed de- sign. Both selection and placement of these capacitors can be critical and, to a large extent, dependent upon the specifics of the system configuration. The dominant consideration in the selection of bypass capacitors for the AD668 is minimization of series resistance and inductance. Many capacitors will begin to look inductive at 20 MHz and above. Ceramic and film type capacitors generally feature lower series inductance than tanta- lum or electrolytic types. A few general rules are of universal use when approaching the problem of bypassing. Bypass capacitors should be installed on the printed circuit board with the shortest possible leads consistent with reliable construction. This helps to minimize series inductance in the leads. Chip capacitors are optimal in this respect. Some series inductance between the DAC supply pins and the power supply plane may help to filter-out high frequency power supply noise. This inductance can be generated using a small ferrite bead. |
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