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AD8184AR-REEL 데이터시트(PDF) 8 Page - Analog Devices |
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AD8184AR-REEL 데이터시트(HTML) 8 Page - Analog Devices |
8 / 12 page AD8184 –8– REV. 0 Chip capacitors should be used for supply bypassing. One end of the capacitor should be connected to the ground plane and the other within 1/4 inch of each power pin. An additional large (4.7 µF–10 µF) tantalum capacitor should be connected in par- allel with each of the smaller capacitors for low impedance sup- ply bypassing over a broad range of frequencies. Signal traces should be as short as possible. Stripline or microstrip techniques should be used for long (longer than about 1 inch) signal traces. These should be designed with a characteristic impedance of 50 Ω or 75 Ω and be properly ter- minated at each end using surface mount components. Careful layout is imperative to minimize crosstalk. Guards (ground or supply traces) must be run between all signal traces to limit direct capacitive coupling. Input and output signal lines should fan out away from the mux as much as possible. If mul- tiple signal layers are available, a buried stripline structure hav- ing ground plane above, below and between signal traces will have the best crosstalk performance. Return currents flowing through termination resistors can also increase crosstalk if these currents flow in sections of the finite- impedance ground circuit shared between more than one input or output. Minimizing the inductance and resistance of the ground plane can reduce this effect, but further care should be taken in po- sitioning the terminations. Terminating cables directly at the con- nectors will minimize the return current flowing on the board, but the signal trace between the connector and the mux will look like an open stub and will degrade the frequency response. Moving the termination resistors close to the input pins will improve the fre- quency response, but the terminations from neighboring inputs should not have a common ground return. APPLICATIONS A Buffered 4-to-1 Multiplexer In applications where the output of a multiplexer must drive a back-terminated 75 Ω line (R L = 75 Ω + 75 Ω), it is necessary to buffer the output of the AD8184. In the example in Figure 22, this is accomplished using the AD8009 high speed current feedback op amp. The amplifier is configured with a gain of 2 to compensate for the signal halving due to termination at the multi- plexer input. This gives the overall circuit a gain of unity. If lower speed can be tolerated, a number of other amplifiers can replace the AD8009 op amp in the above circuit. In general there is a trade-off between bandwidth and power consumption. Table II summarizes the bandwidth and power consumption characteristics of these op amps. Table II. Amplifier Options for Multiplexer Buffering Op Amp Comments AD8009 Highest Bandwidth, (G = +2) = 700 MHz, ISY = 14 mA AD8001 Lower Power Consumption, Bandwidth (G = +2) = 440 MHz, ISY = 5 mA AD8011 Lower Power Consumption, Bandwidth (G = +2) = 210 MHz, ISY = 1 mA AD8079 Fixed Gain Dual Amplifier (2 or 2.2), Bandwidth = 260 MHz, ISY = 5 mA Per Amp AD8005 Lowest Power Consumption, Bandwidth (G = +2) = 170 MHz, ISY = 400 µA –VS 681 Ω +1 DECODER 1 2 3 4 5 6 7 14 13 12 11 10 9 8 +1 +1 +1 AD8184 75 Ω 75 Ω 75 Ω 75 Ω +VS NC –VS GND GND GND 0.1µF 10µF VOUT 0.1µF 10µF 10µF 0.1µF +VS +VS 10µF 0.1µF A0 A1 75 Ω –VS 681 Ω AD8009 IN0 IN1 IN2 IN3 Figure 22. A Buffered 4-to-1 Multiplexer |
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