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AD8131ARM-REEL Datasheet(PDF) 9 Page - Analog Devices |
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AD8131ARM-REEL Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page REV. 0 AD8131 –9– OPERATIONAL DESCRIPTION Definition of Terms AD8131 +IN –IN RF RF RG RG +DIN VOCM –DIN RL,dm +OUT VOUT,dm –OUT +OUT –OUT Figure 37. 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: VOUT,dm = (V+OUT – V–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 volt- ages. The output common-mode voltage is defined as: VOUT,cm = (V+OUT + V–OUT)/2 Balance is a measure of how well differential signals are matched in amplitude and exactly 180 degrees apart in phase. Balance is most easily determined by placing a well-matched resistor divider between the differential voltage nodes and comparing the magnitude of the signal at the divider’s midpoint with the magnitude of the differential signal. 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 AD8131 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 feed- back to force these outputs to the desired voltages. The AD8131 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. 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 AD8131 uses two feedback loops to separately control the differential and common-mode output voltages. The differential feedback, set by internal 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 internal common-mode feedback, to be equal to the voltage applied to the VOCM input, without affecting the differential output voltage. The AD8131 architecture results in outputs that are very highly balanced over a wide frequency range without requiring external components or adjustments. 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 degrees apart in phase. Analyzing an Application Circuit The AD8131 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 37. For most purposes, this voltage can be assumed to be zero. Similarly, 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. Closed-Loop Gain The differential mode gain of the circuit in Figure 37 can be determined to be described by the following equation: V V R R OUT dm IN dm F G , , == 2 where RF = 1.5 k Ω and R G = 750 Ω nominally. 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 =+ = 13 The total output referred noise for the AD8131, including the contributions of RF, RG, and op amp, is nominally 25 nV/√Hz at 20 MHz. Calculating an Application Circuit’s Input Impedance The effective input impedance of a circuit such as that in Figure 37, at +DIN and –DIN, will depend on whether the amplifier is being driven by a single-ended or differential signal source. For balanced differential input signals, the input impedance (RIN,dm) between the inputs (+DIN and –DIN) is simply: RIN,dm = 2 × RG = 1.5 kΩ In the case of a single-ended input signal (for example if –DIN is grounded and the input signal is applied to +DIN), the input impedance becomes: |
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