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AD8432ACPZ-R7 数据表(PDF) 19 Page - Analog Devices |
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AD8432ACPZ-R7 数据表(HTML) 19 Page - Analog Devices |
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19 / 28 page  AD8432 Rev. 0 | Page 19 of 28 The single-ended gain from INH to OPH (see Figure 60) is defined as G1 G4 G3 G2 G1 INH OPH R R R R R G + + + = − The single-ended gain from INH to OPL is defined as G1 G7 G6 G5 INH OPL R R R R G + + − = − The values of the seven gain resistors were chosen so that both single-ended gains are equal. For example, to set a gain of 12.04 dB (G = ×4) differentially, the gain from INH to each output (OPH, OPL) should be 6.02 dB (G = ×2). INH to OPH: For RG1 = RG2 = RG, then 2 2 = × = + = − G G G1 G2 G1 INH OPH R R R R R G INH to OPL: For RG1 = RG and RG5 = 2 × RG, then 2 2 − = × − = − = − G G G1 G5 INH OPL R R R R G ACTIVE INPUT RESISTANCE MATCHING The AD8432 reduces noise and optimizes signal power transfer by using active input termination to perform signal source resistance matching. The primary purpose of input impedance matching is to optimize the input signal power transfer. With resistive termination, the input noise increases due to the thermal noise of the terminating resistor and the increased contribution of the input voltage noise generator of the LNA. With active impedance matching, however, the contributions of both are smaller than they would be for resistive termination by a factor of 1/(1 + ½LNA Gain). The noise figure (NF) for the three terminating schemes are shown in Figure 62. LNA RIN VIN VOUT RS INH UNTERMINATED LNA RIN VIN VOUT RS RS INH RESISTIVE TERMINATION LNA RIN VIN VOUT RS RFB INH ACTIVE IMPEDANCE MATCH Figure 61. Input Resistance Matching To achieve this active impedance match, connect a feedback resistor RFB between the INH and OPL (see Figure 61). RIN is given in Equation 1, where G/2 is the single-ended gain. 2 1 G R R FB IN + = (1) In addition, to further reduce the input resistance, there is an internal resistance of 6.2 kΩ in parallel with the source resistance, such that INTERNAL FB IN R G R R 2 1 + = (2) Equation 3 should be used to calculate RFB accurately for a desired input resistance and single-ended gain. Refer to Table 6 for calculated results for RFB for several input resistance and gain combinations. kΩ 2 . 6 , 1 2 1 = − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + = ⇒ INTERNAL INTERNAL IN IN FB R R R G R R (3) 0 1 2 3 4 5 6 7 8 100 50 1000 RS (Ω) (SIMULATED RESULTS) ACTIVE IMPEDANCE MATCH UNTERMINATED RESISTIVE TERMINATION (RS = RIN) Figure 62. Noise Figure vs. RS for Resistive, Active Match, and Unterminated Inputs 0 2 6 4 8 10 12 14 16 18 100 50 1k RS (Ω) (SIMULATED RESULTS) RIN = UNTERMINATED RIN = 50Ω RIN = 75Ω RIN = 100Ω RIN = 200Ω RIN = 1kΩ Figure 63. Noise Figure vs. RS for Various Values of RIN, Actively Matched |
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