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AD8011ARZ-REEL Datasheet(PDF) 10 Page - Analog Devices |
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AD8011ARZ-REEL Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page REV. C –10– AD8011 (error current times the open-loop inverting input resistance) that results (see Figure 7), a more exact low frequency closed-loop transfer function can be described as A G GR T R T G G A R T V I O F OO F O = + × + = ++ 11 for noninverting (G is positive). A V O F O G G A R T = ++ 1 1 – for inverting (G is negative). where G is the ideal gain as previously described. With RI = TO /AO (open-loop inverting input resistance), the second expression (positive G) clearly relates to the classical voltage feedback op amp equation with TO omitted due to its relatively much higher value and thus insignificant effect. AO and TO are the open-loop dc voltage and transresistance gains of the amplifier, respectively. These key transfer variables can be described as A Rgmf A g mc R O = ×× × 12 1 1 | | ( – ) and T RA O = × 12 2 | | Therefore R g mc R g mf I = × × 1 1 2 – where gmc is the positive feedback transconductance (not shown) and 1/gmf is the thermal emitter resistance of devices D1/D2 and Q3/Q4. The gmc × R1 product has a design value that results in a negative dc open-loop gain of typically –2500 V/V (see Figure 8). RS LN TO (s) AO (s) VP ZI IE LI RN CP RF +VS –VS LS RL CL VO LS Z I = OPEN LOOP INPUT IMPEDANCE = CI || RL Figure 7. ZI = Open-Loop Input Impedance Though atypical of conventional CF or VF amps, this negative open-loop voltage gain results in an input referred error term (VP–VO/G = G/AO + RF/TO) that will typically be negative for G, greater than +3/–4. As an example, for G = 10, AO = –2500, and TO = 1.2 M Ω, results in an error of –3 mV using the A V derivation above. This analysis assumes perfect current sources and infinite transistor VAs. (Q3, Q4 output conductances are assumed zero.) These assumptions result in actual versus model open-loop voltage gain and associated input referred error terms being less accurate for low gain (G) noninverting operation at the frequencies below the open-loop pole of the AD8011. This is primarily a result of the input signal (VP) modulating the output conductances of Q3/Q4, resulting in RI less negative than derived here. For inverting operation, the actual versus model dc error terms are relatively much less. 1E+03 1E+04 1E+05 1E+06 1E+07 1E+08 1E+09 80 70 60 50 40 20 10 FREQUENCY (Hz) 30 –90 –100 –110 –120 –160 PHASE GAIN 0 –10 –20 –30 –170 –180 –190 –200 –130 –140 –150 AO(s) Figure 8. Open-Loop Voltage Gain and Phase AC TRANSFER CHARACTERISTICS The ac small signal transfer derivations below are based on a simplified single-pole model. Though inaccurate at frequencies approaching the closed-loop BW (CLBW) of the AD8011 at low noninverting external gains, they still provide a fair approxima- tion and an intuitive understanding of its primary ac small signal characteristics. For inverting operation and high noninverting gains, these transfer equations provide a good approximation to the actual ac performance of the device. To accurately quantify the VO versus VP relationship, AO(s) and TO(s) need to be derived. This can be seen by the following nonexpanded noninverting gain relationship Vs V s G G As R Ts OP O F O () / () [] [] = ++ 1 with As Rg mf A g mc R S g mc R O () || – – = ×× × × 12 11 1 11 τ where R1 is the input resistance to A2/A2B, and τ1 (equal to CD R1 A2) is the open-loop dominate time constant, and T s AR s O () || = × + 21 2 11 τ |
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