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AD712JR-REEL7 Datenblatt(PDF) 8 Page - Analog Devices |
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AD712JR-REEL7 Datenblatt(HTML) 8 Page - Analog Devices |
8 / 15 page AD712 REV. B –8– OP AMP SETTLING TIME - A MATHEMATICAL MODEL The design of the AD712 gives careful attention to optimizing individual circuit components; in addition, a careful trade-off was made: the gain bandwidth product (4 MHz) and slew rate (20 V/ µs) were chosen to be high enough to provide very fast settling time but not too high to cause a significant reduction in phase margin (and therefore stability). Thus designed, the AD712 settles to ±0.01%, with a 10 V output step, in under 1 µs, while retaining the ability to drive a 250 pF load capaci- tance when operating as a unity gain follower. If an op amp is modeled as an ideal integrator with a unity gain crossover frequency of ωο/2π, Equation 1 will accurately de- scribe the small signal behavior of the circuit of Figure 26a, consisting of an op amp connected as an I-to-V converter at the output of a bipolar or CMOS DAC. This equation would com- pletely describe the output of the system if not for the op amp’s finite slew rate and other nonlinear effects. Equation 1. V O I IN = – R R(C f = CX ) ωο s 2 + G N ωο + RC f s +1 where ωο 2π =op amp’s unity gain frequency GN = “noise” gain of circuit 1 + R R O This equation may then be solved for Cf: Equation 2. C f = 2 − G N R ωο + 2 RC X ω ο + (1 − GN ) R ωο In these equations, capacitor CX is the total capacitor appearing the inverting terminal of the op amp. When modeling a DAC buffer application, the Norton equivalent circuit of Figure 26a can be used directly; capacitance CX is the total capacitance of the output of the DAC plus the input capacitance of the op amp (since the two are in parallel). 1/2 AD712 VOUT RL CL CF R IO RO CX Figure 26a. Simplified Model of the AD712 Used as a Current-Out DAC Buffer When RO and IO are replaced with their Thevenin VIN and RIN equivalents, the general purpose inverting amplifier of Figure 26b is created. Note that when using this general model, capaci- tance CX is EITHER the input capacitance of the op amp if a simple inverting op amp is being simulated OR it is the com- bined capacitance of the DAC output and the op amp input if the DAC buffer is being modeled. 1/2 AD712 VOUT RL CL CF R VIN RIN CX Figure 26b. Simplified Model of the AD712 Used as an Inverter In either case, the capacitance CX causes the system to go from a one-pole to a two-pole response; this additional pole increases settling time by introducing peaking or ringing in the op amp output. Since the value of CX can be estimated with reasonable accuracy, Equation 2 can be used to choose a small capacitor, CF, to cancel the input pole and optimize amplifier response. Figure 27 is a graphical solution of Equation 2 for the AD712 with R = 4 k Ω. CF 40 30 0 10 0 20 10 GN = 3.0 GN = 2.0 GN = 1.5 GN = 1.0 20 30 40 50 60 50 60 GN = 4.0 Figure 27. Value of Capacitor CF vs. Value of CX |
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Ähnliche Beschreibung - AD712JR-REEL7 |
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