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ADA4930-2YCPZ-R7 数据表(PDF) 20 Page - Analog Devices |
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ADA4930-2YCPZ-R7 数据表(HTML) 20 Page - Analog Devices |
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20 / 25 page  ADA4930-1/ADA4930-2 Data Sheet Rev. B | Page 20 of 25 Terminating a Single-Ended Input This section describes the five steps that properly terminate a single-ended input to the ADA4930-1/ADA4930-2. Assume a system gain of 1, RF1 = RF2 = 301 Ω, an input source with an open- circuit output voltage of 2 V p-p, and a source resistance of 50 Ω. Figure 46 shows this circuit. 1. Calculate the input impedance. β1 = β2 = 301/602 = 0.5 and RIN = 401.333 Ω RS 50Ω VS 2V p-p RIN 401.333Ω ADA4930 RL VOUT, dm +VS –VS RG1 301Ω RG2 301Ω RF2 301Ω RF1 301Ω VOCM Figure 46. Single-Ended Input Impedance RIN 2. Add a termination resistor, RT. To match the 50 Ω source resistance, RT is added. Because RT||401.33 Ω = 50 Ω, RT = 57.116 Ω. ADA4930 RL VOUT, dm +VS –VS RS 50Ω RG1 301Ω RG2 301Ω RF2 301Ω RF1 301Ω VOCM VS 2V p-p RIN 50Ω RT 57.116Ω Figure 47. Adding Termination Resistor RT 3. Replace the source-termination resistor combination with its Thevenin equivalent. The Thevenin equivalent of the source resistance RS and the termination resistance RT is RTH = RS||RT = 26.66 Ω. The Thevenin equivalent of the source voltage is VTH = VS T S T R R R = 1.066 V p-p RS 50Ω VS 2V p-p RT 57.116Ω RTH 26.661Ω VTH 1.066V p-p Figure 48. Thevenin Equivalent Circuit 4. Set RF1 = RF2 = RF to maintain a balanced system. Compensate the imbalance caused by RTH. There are two methods available to compensate, which follow: Add RTH to RG2 to maintain balanced gain resistances and increase RF1 and RF2 to RF = TH S V V Gain(RG + RTH) to maintain the system gain. Decrease RG2 to RG2 = Gain V V R S TH F to maintain system gain and decrease RG1 to (RG2 − RTH) to maintain balanced gain resistances. The first compensation method is used in the Analog Devices DiffAmpCalc™ tool. Using the second compensation method, RG2 = 160.498 Ω and RG1 = 160.498 − 26.66 = 133.837 Ω. The modified circuit is shown in Figure 49. ADA4930 RL VOUT, dm +VS –VS RTH 26.661Ω RG1 133.837Ω RG2 160.498Ω RF2 301Ω RF1 301Ω VOCM VTH 1.066V p-p Figure 49. Thevenin Equivalent with Matched Gain Resistors Figure 49 presents an easily manageable circuit with matched feedback loops that can be easily evaluated. 5. The modified gain resistor, RG1, changes the input impedance. Repeat Step 1 through Step 4 several times using the modified value of RG1 from the previous iteration until the value of RT does not change from the previous iteration. After three additional iterations, the change in RG1 is less than 0.1%. The final circuit is shown in Figure 50 with the closest 0.5% resistor values. ADA4930 RL VOUT, dm 1.990V p-p +VS –VS RS 50Ω RG 142Ω VP VN RG2 169Ω RF1 301Ω RF2 301Ω VOCM VS 2V p-p 0.998V p-p RT 64.2Ω Figure 50. Terminated Single-Ended-to-Differential System with G = 1 |
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