Datasheet AD8004 (Analog Devices) - 9

AD8004 THEORY OF OPERATION
The more exact relationships that take into account open-loop The AD8004 is a member of a new family of high speed current- gain errors are: feedback (CF) amplifiers offering new levels of bandwidth, distortion, and signal-swing capability vs. power. Its wide dynamic A = G range capabilities are due to both a complementary high speed V 1 + 1 − G + RF for inverting (G is negative) bipolar process and a new design architecture. The AD8004 is A (s) T (s) O O basically a two stage (Figure 30) rather than the conventional one stage design. Both stages feature the current-on-demand property associated with current feedback amplifiers. This A = G V gives an unprecedented ratio of quiescent current to dynamic 1 + G + RF for noninverting (G is positive) performance. The important properties of slew rate and full A (s) T (s) O O power bandwidth benefit from this performance. In addition In these equations the open-loop voltage gain (AO(s)) is common the second gain stage buffers the effects of load impedance, to both voltage and current-feedback amplifiers and is the ratio significantly reducing distortion. of output voltage to differential input voltage. The open-loop A full discussion of this new amplifier architecture is available on transimpedance gain (TO(s)) is the ratio of output voltage to the data sheet for the AD8011. This discussion only covers the inverting input current and is applicable to current-feedback basic principles of operation. amplifiers. The open-loop voltage gain and open-loop transim- pedance gain (TO(s)) of the AD8004 are plotted vs. frequency
DC AND AC CHARACTERISTICS
in TPCs 15 and 18. These plots and the basic relationships can As with traditional op amp circuits the dc closed-loop gain is be used to predict the first order performance of the AD8004 over defined as: frequency. At low closed-loop gains the term (RF/TO(s)) dominates the frequency response characteristics. This gives the result that bandwidth is constant with gain, a familiar property of current A = G = 1+ RF V noninverting operation RN feedback amplifiers. An RF of 1 k⍀ has been chosen as the nominal value to give A = G = − RF optimum frequency response with acceptable peaking at gains of V inverting operation RN +2/–1. As can be seen from the above relationships, at higher closed-loop gains reducing RF has the effect of increasing closed- loop bandwidth. Table I gives optimum values for RF and RG for a variety of gains.
A1 CD IPP IPN IQ1 A2 CP1 Q3 CP2 ICQ + IO Q1 V V P N V Z O´ I A3 VO RL CL Q2 Z2 RF IE RG Q4 IQ1 A2 CP1 INP IPN A1 CD AD8004
Figure 5. Simplified Block Diagram REV. D –9– Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION CONNECTION DIAGRAM SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE MAXIMUM POWER DISSIPATION Typical Performance Characteristics THEORY OF OPERATION DC AND AC CHARACTERISTICS DRIVING CAPACITIVE LOADS OPTIMIZING FLATNESS DRIVING A SINGLE-SUPPLY A/D CONVERTER LAYOUT CONSIDERATIONS OUTLINE DIMENSIONS Revision History