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AD8021 数据表(PDF) 19 Page - Analog Devices |
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AD8021 数据表(HTML) 19 Page - Analog Devices |
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19 / 28 page  ADA4896-2/ADA4897-1 Rev. 0 | Page 19 of 28 DC ERRORS Figure 45 shows a typical connection diagram and the major dc error sources. RG – VIN + RS – VIP + IB+ IB– + VOUT – RF + VOS – Figure 45. Typical Connection Diagram and DC Error Sources The ideal transfer function (all error sources set to 0 and infinite dc gain) can be written as IN G F IP G F OUT V R R V R R V × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + = 1 (1) This reduces to the familiar forms for inverting and noninverting op amp gain expressions, as follows: (Noninverting gain, VIN = 0 V) IP G F OUT V R R V × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + = 1 (2) (Inverting gain, VIP = 0 V) IN G F OUT V R R V × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − = (3) The total output voltage error is the sum of errors due to the amplifier offset voltage and input currents. The output error due to the offset voltage can be estimated as ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + − + + = G F OUT PNOM P OFFSET OUT R R A V PSRR V V CMRR VCM V V NOM ERROR 1 (4) where: is the offset voltage at the specified supply voltage, which is measured with the input and output at midsupply. VCM is the common-mode voltage. VP is the power supply voltage. VPNOM is the specified power supply voltage. CMRR is the common-mode rejection ratio. PSRR is the power supply rejection ratio. A is the dc open-loop gain. NOM OFFSET V The output error due to the input currents can be estimated as + − × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + × − ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + × = B G F S B G F G F OUT I R R R I R R R R V ERROR 1 1 ) || ( (5) Note that setting RS equal to RF||RG compensates for the voltage error due to the input bias current. NOISE CONSIDERATIONS Figure 46 illustrates the primary noise contributors for the typical gain configurations. The total rms output noise is the root-mean-square of all the contributions. RG RS iep ien + vout_en – RF ven 4kT × RS vn _ RS = 4kT × RG vn _ RG = 4kT × RF vn _ RF = Figure 46. Noise Sources in Typical Connection The output noise spectral density can be calculated by [] 2 2 2 2 2 2 2 4 4 1 4 _ F G G F S G F F R ien kTR R R ven R iep kTRs R R kTR en vout + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + + + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + + = (6) where: k is Boltzmann’s Constant. T is the absolute temperature, degrees Kelvin. ien is the amplifier input current noise spectral density, pA/√Hz. ven is the amplifier input voltage spectral density, nV/√Hz. RS is the source resistance, as shown in . RF and RG are the feedback network resistances, as shown in . Figure 46 Figure 46 Source resistance noise, amplifier voltage noise (ven), and the voltage noise from the amplifier current noise (iep × RS) are all subject to the noise gain term (1 + RF/RG). Note that with a 1 nV/√Hz input voltage noise and 2.8 pA/√Hz input current, the noise contributions of the amplifier are relatively small for source resistances between approximately 50 Ω and 700 Ω. shows the total RTI noise due to the amplifier vs. the source resistance. In addition, the value of the feedback resistors used impacts the noise. It is recommended that the value of the feedback resistors be maintained between 250 Ω and 1 kΩ to keep the total noise low. Figure 47 50 500 SOURCE RESISTANCE (Ω) 5 0.5 50 500 5k 50k TOTAL AMPLIFIER NOISE AMPLIFIER AND RESISTOR NOISE SOURCE RESISTANCE NOISE Figure 47. RTI Noise vs. Source Resistance |
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