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ADA4895-2ARMZ-R7 数据表(PDF) 18 Page - Analog Devices |
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ADA4895-2ARMZ-R7 数据表(HTML) 18 Page - Analog Devices |
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18 / 24 page ![]() ADA4895-2 Data Sheet Rev. 0 | Page 18 of 24 DC ERRORS Figure 39 shows a typical connection diagram and the major dc error sources. RG – VIN + RS – VIP + IB+ IB– + VOUT – RF + VOS – Figure 39. Typical Connection Diagram and DC Error Sources The ideal transfer function (all error sources set to 0 and infinite dc gain) can be expressed as follows: IN G F IP G F OUT V R R V R R V × − × + = 1 (1) This equation reduces to the familiar forms for noninverting and inverting op amp gain expressions, as follows: For noninverting gain (VIN = 0 V), IP G F OUT V R R V × + = 1 (2) For inverting gain (VIP = 0 V), IN G F OUT V R R V × − = (3) The total output voltage error is the sum of the errors due to the amplifier offset voltage and input currents. The output error due to the offset voltage can be estimated as follows: ERROR OUT V = (4) + × + − + + G F OUT PNOM P CM OFFSET R R A V PSRR V V CMRR V V NOM 1 where: NOM OFFSET V 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. CMRR is the common-mode rejection ratio. VP is the power supply voltage. VPNOM is the specified power supply voltage. PSRR is the power supply rejection ratio. A is the dc open-loop gain. The output error due to the input currents can be estimated as follows: + − × + × − × + × = B G F S B G F G F OUT I R R R I R R R R V ERROR 1 1 ) || ( (5) BIAS CURRENT CANCELLATION To cancel the output voltage error due to unmatched bias currents at the inputs, Resistors RBP and RBN can be used (see Figure 40). RG RS RBP RBN RF Figure 40. Using RBP and RBN to Cancel Bias Current Error To compensate for the unmatched bias currents at the two inputs, set Resistors RBP and RBN as shown in Table 8. Table 8. Setting RBP and RBN to Cancel Bias Current Error Value of R F||RG Value of R BP (Ω) Value of R BN (Ω) Greater Than R S R F||RG − RS 0 Less Than R S 0 R S − RF||RG |
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