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AD8056ARM-REEL Datasheet(PDF) 9 Page - Analog Devices |
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AD8056ARM-REEL Datasheet(HTML) 9 Page - Analog Devices |
9 / 11 page AD8055/AD8056 –9– REV. B APPLICATIONS Four-Line Video Driver The AD8055 is a useful low cost circuit for driving up to four video lines. For such an application, the amplifier is configured for a noninverting gain of 2 as shown in Figure 33. The input video source is terminated in 75 Ω and applied to the high impedance noninverting input. Each output cable is connected to the op amp output via a 75 Ω series back termination resistor for proper cable termination. The terminating resistors at the other ends of the lines will divide the output signal by two, which is compensated for by the gain-of-two of the op amp stage. For a single load, the differential gain error of this circuit was measured to be 0.01%, with a differential phase error of 0.02 degrees. The two load measurements were 0.02% and 0.03 degrees, respectively. For four loads, the differential gain error is 0.02%, while the differential phase increases to 0.1 degrees. 0.1 F 10 F 75 402 AD8055 75 +5V –5V 0.1 F 10 F 402 75 75 75 VIN 75 75 75 75 VOUT1 VOUT2 VOUT3 VOUT4 6 7 2 3 4 Figure 33. Four-Line Video Driver Single-Ended to Differential Line Driver Creating differential signals from single-ended signals is required for driving balanced, twisted pair cables, differential input A/D converters and other applications that require differen- tial signals. This is sometimes accomplished by using an inverting and a noninverting amplifier stage to create the complementary signals. The circuit shown in Figure 34 shows how an AD8056 can be used to make a single-ended to differential converter that offers some advantages over the architecture mentioned above. Each op amp is configured for unity gain by the feedback resistors from the outputs to the inverting inputs. In addition, each out- put drives the opposite op amp with a gain of –1 by means of the crossed resistors. The result of this is that the outputs are comple- mentary and there is high gain in the overall configuration. Feedback techniques similar to a conventional op amp are used to control the gain of the circuit. From the noninverting input of Amp 1 to the output of Amp 2, is an inverting gain. Between these points a feedback resistor can be used to close the loop. As in the case of a conventional op amp inverting gain stage, an input resistor is added to vary the gain. The gain of this circuit from the input to Amp 1 output is RF/RI, while the gain to the output of Amp 2 is –RF/RI. The circuit thus creates a balanced differential output signal from a single- ended input. The advantage of this circuit is that the gain can be changed by changing a single resistor and still maintain the balanced differential outputs. 75 RI 402 +5V RF 402 –5V AD8056 402 402 402 49.9 49.9 VIN +VOUT 402 –VOUT 10 F 0.1 F 1 2 3 8 AMP1 5 6 7 4 AMP2 10 F 0.1 F Figure 34. Single-Ended to Differential Line Driver Low Noise, Low Power Preamp The AD8055 makes a good low cost, low noise, low power preamp. A gain of 10 preamp can be made with a feedback resistor of 909 ohms and a gain resistor of 100 ohms as shown in Figure 35. The circuit has a –3 dB bandwidth of 20 MHz. 0.1 F 10 F +5V –5V 0.1 F10 F 909 VOUT + 100 RS AD8055 6 7 2 3 4 Figure 35. Low Noise, Low Power Preamp with G = 10 and BW = 20 MHz With a low source resistance (<approximately 100 Ω), the major contributors to the input referred noise of this circuit are the input voltage noise of the amplifier and the noise of the 100 Ω resistor. These are 6 nV/ √Hz and 1.2 nV/√Hz, respectively. These values yield a total input referred noise of 6.1 nV/ √Hz. |
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