link to page 12 link to page 12 link to page 12 AD8055/AD8056APPLICATIONS FOUR-LINE VIDEO DRIVER Between these points, a feedback resistor can be used to close the loop. As in the case of a conventional op amp inverting gain The AD8055 is a useful low cost circuit for driving up to four stage, an input resistor is added to vary the gain. video lines. For such an application, the amplifier is configured for a noninverting gain of 2, as shown in Figure 36. The input The gain of this circuit from the input to AMP1 output is RF/RI, video source is terminated in 75 Ω and is applied to the high while the gain to the output of AMP2 is −RF /RI. The circuit impedance noninverting input. therefore creates a balanced differential output signal from a single-ended input. The advantage of this circuit is that the gain Each output cable is connected to the op amp output via a 75 Ω can be changed by changing a single resistor, while still series back termination resistor for proper cable termination. maintaining the balanced differential outputs. The terminating resistors at the other ends of the lines divide the output signal by 2, which is compensated for by the gain of 2 RF402Ω of the op amp stage. +5V For a single load, the differential gain error of this circuit was 0.1µF10µF measured as 0.01%, with a differential phase error of 0.02°. The RI402Ω8 two load measurements were 0.02% and 0.03°, respectively. For V3IN49.9ΩAMP11+VOUT four loads, the differential gain error is 0.02%, while the 2 differential phase increases to 0.1°. 402Ω75Ω+5VVOUT175Ω402Ω402ΩAD8056402Ω75Ω0.1µF10µF402ΩV402ΩOUT22775ΩAD8055 6V346IN49.9Ω75ΩAMP27–VOUT75ΩVOUT30.1µF10µF5475Ω75Ω0.1µF10µF 37 0 –5V75Ω 3- VOUT4–5V 06 01 75Ω 036 63- Figure 37. Single-Ended-to-Differential Line Driver 010 Figure 36. Four-Line Video Driver LOW NOISE, LOW POWER PREAMPSINGLE-ENDED-TO-DIFFERENTIAL LINE DRIVER The AD8055 makes a good, low cost, low noise, low power preamp. A gain-of-10 preamp can be made with a feedback Creating differential signals from single-ended signals is resistor of 909 Ω and a gain resistor of 100 Ω, as shown in required for driving balanced, twisted pair cables, differential input ADCs, and other applications that require differential Figure 38. The circuit has a −3 dB bandwidth of 20 MHz. signals. This can be accomplished by using an inverting and a 909Ω noninverting amplifier stage to create the complementary +5V signals. +0.1µF10µF100Ω27 The circuit shown in Figure 37 shows how an AD8056 can be AD8055 6VOUT used to make a single-ended-to-differential converter that offers 34 some advantages over the architecture previously mentioned. RS Each op amp is configured for unity gain by the feedback 0.1µF10µF 8 resistors from the outputs to the inverting inputs. In addition, -03 –5V 63 10 each output drives the opposite op amp with a gain of −1 by 0 means of the crossed resistors. The result of this is that the Figure 38. Low Noise, Low Power Preamp with G = +10 and BW = 20 MHz outputs are complementary and there is high gain in the overall With a low source resistance (< approximately 100 Ω), the configuration. major contributors to the input-referred noise of this circuit are Feedback techniques similar to a conventional op amp are used the input voltage noise of the amplifier and the noise of the to control the gain of the circuit. From the noninverting input 100 Ω resistor. These are 6 nV/√Hz and 1.2 nV/√Hz, respectively. of AMP1 to the output of AMP2 is an inverting gain. These values yield a total input referred noise of 6.1 nV/√Hz. Rev. J | Page 12 of 16 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION CONNECTION DIAGRAMS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS MAXIMUM POWER DISSIPATION ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS TEST CIRCUITS APPLICATIONS FOUR-LINE VIDEO DRIVER SINGLE-ENDED-TO-DIFFERENTIAL LINE DRIVER LOW NOISE, LOW POWER PREAMP POWER DISSIPATION LIMITS RESISTOR SELECTION DRIVING CAPACITIVE LOADS OUTLINE DIMENSIONS ORDERING GUIDE
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