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AD8138AR датащи(PDF) 9 Page - Analog Devices |
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AD8138AR датащи(HTML) 9 Page - Analog Devices |
9 / 16 page REV. E AD8138 –9– OPERATIONAL DESCRIPTION Definition of Terms AD8138 CF +IN –IN RF CF RF RG RG +DIN VOCM –DIN RL,dm +OUT VOUT,dm –OUT Figure 2. Circuit Definitions Differential voltage refers to the difference between two node voltages. For example, the output differential voltage (or equivalently output differential-mode voltage) is defined as: VV V dm OUT OUT OUT, =- () +- V+OUT and V–OUT refer to the voltages at the +OUT and –OUT terminals with respect to a common reference. Common-mode voltage refers to the average of two node voltages. The output common-mode voltage is defined as: VV V cm OUT OUT OUT, =+ () +- 2 Balance is a measure of how well differential signals are matched in amplitude and exactly 180 apart in phase. Balance is most easily determined by placing a well-matched resistor divider between the differential voltage nodes and comparing the magni- tude of the signal at the divider’s midpoint with the magnitude of the differential signal (see TPC 26). By this definition, output balance is the magnitude of the output common-mode voltage divided by the magnitude of the output differential-mode voltage: Output Balance Error V V OUT cm OUT dm = , , THEORY OF OPERATION The AD8138 differs from conventional op amps in that it has two outputs whose voltages move in opposite directions. Like an op amp, it relies on high open-loop gain and negative feedback to force these outputs to the desired voltages. The AD8138 behaves much like a standard voltage feedback op amp and makes it easy to perform single-ended-to-differential conversion, common- mode level-shifting, and amplification of differential signals. Also like an op amp, the AD8138 has high input impedance and low output impedance. Previous differential drivers, both discrete and integrated designs, have been based on using two independent amplifiers and two independent feedback loops, one to control each of the outputs. When these circuits are driven from a single-ended source, the resulting outputs are typically not well balanced. Achieving a balanced output has typically required exceptional matching of the amplifiers and feedback networks. DC common-mode level-shifting has also been difficult with previous differential drivers. Level-shifting has required the use of a third amplifier and feedback loop to control the output common-mode level. Sometimes the third amplifier has also been used to attempt to correct an inherently unbalanced circuit. Excellent performance over a wide frequency range has proven difficult with this approach. The AD8138 uses two feedback loops to separately control the differential and common-mode output voltages. The differential feedback, set with external resistors, controls only the differential output voltage. The common-mode feedback controls only the common-mode output voltage. This architecture makes it easy to arbitrarily set the output common-mode level. It is forced, by inter- nal common-mode feedback, to be equal to the voltage applied to the VOCM input, without affecting the differential output voltage. The AD8138 architecture results in outputs that are very highly balanced over a wide frequency range without requiring tightly matched external components. The common-mode feedback loop forces the signal component of the output common-mode voltage to be zeroed. The result is nearly perfectly balanced differential outputs of identical amplitude and exactly 180 ∞ apart in phase. Analyzing an Application Circuit The AD8138 uses high open-loop gain and negative feedback to force its differential and common-mode output voltages in such a way as to minimize the differential and common-mode error voltages. The differential error voltage is defined as the voltage between the differential inputs labeled +IN and –IN in Figure 2. For most purposes, this voltage can be assumed to be zero. Simi- larly, the difference between the actual output common-mode voltage and the voltage applied to VOCM can also be assumed to be zero. Starting from these two assumptions, any application circuit can be analyzed. Setting the Closed-Loop Gain Neglecting the capacitors CF, the differential-mode gain of the circuit in Figure 2 can be determined to be described by the following equation: V V R R OUT dm IN dm F S G S , , = This assumes the input resistors, RG S, and feedback resistors, RF S, on each side are equal. Estimating the Output Noise Voltage Similar to the case of a conventional op amp, the differential output errors (noise and offset voltages) can be estimated by multiplying the input referred terms, at +IN and –IN, by the circuit noise gain. The noise gain is defined as: G R R N F G =+ Ê ËÁ ˆ ¯˜ 1 To compute the total output referred noise for the circuit of Figure 2, consideration must also be given to the contribution of the resistors RF and RG. Refer to Table I for estimated output noise voltage densities at various closed-loop gains. Table I. RG RF Bandwidth Output Noise Output Noise Gain ( )( ) –3 dB 8138 Only 8138 + RG, RF 1 499 499 320 MHz 10 nV/ ÷Hz 11.6 nV/ ÷Hz 2 499 1.0 k 180 MHz 15 nV/ ÷Hz 18.2 nV/ ÷Hz 5 499 2.49 k 70 MHz 30 nV/ ÷Hz 37.9 nV/ ÷Hz 10 499 4.99 k 30 MHz 55 nV/ ÷Hz 70.8 nV/ ÷Hz |
Аналогичный номер детали - AD8138AR |
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Аналогичное описание - AD8138AR |
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