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AD744KR-REEL7 Datenblatt(PDF) 8 Page - Analog Devices |
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AD744KR-REEL7 Datenblatt(HTML) 8 Page - Analog Devices |
8 / 12 page REV. C AD744 –8– EXTERNAL FREQUENCY COMPENSATION Even though the AD744 is useable without compensation in most applications, it may be externally compensated for even more flexibility. This is accomplished by connecting a capacitor between Pins 5 and 8. Figure 28, a simplified schematic of the AD744, shows where this capacitor is connected. This feature is useful because it allows the AD744 to be used as a unity gain voltage follower. It also enables the amplifier to drive capacitive loads up to 2000 pF and greater. 5pF +IN 300 –IN NULL / COMPENSATION NULL / DECOMPENSATION 300 COMPENSATION 2mA 400 A +VS OUTPUT –VS 1k 1k 8k Figure 28. AD744 Simplified Schematic The slew rate and gain bandwidth product of the AD744 are in- versely proportional to the value of the compensation capacitor, CCOMP. Therefore, when trying to maximize the speed of the amplifier, the value of CCOMP should be minimized. CCOMP can also be used to slow the amplifier to a point where the slew rate is perfectly symmetrical and well controlled. Figure 29 sum- marizes the effect of external compensation on slew rate and bandwidth. CCOMP – pF 2 0 20 0.02 0.2 10 100 1000 10 100 0.1 1.0 Figure 29. Gain Bandwidth and Slew Rate vs. CCOMP The following section provides tables to show what CCOMP values will provide the necessary compensation for given circuit configurations and capacitive loads. In each case, the recommended CCOMP is a minimum value. A larger CCOMP can always be used, but slew rate and bandwidth performance will be degraded. Figure 30 shows the AD744 configured as a unity gain voltage follower. In this case, a minimum compensation capacitor of 5 pF is necessary for stable operation. Larger compensation ca- pacitors can be used for driving larger capacitive loads. Table I outlines recommended minimum values for CCOMP based on the desired capacitive load. It also gives the slew rate and band- width that will be achieved for each case. AD744 1 F 0.1 F –VS CCOMP 5pF 1 F 0.1 F +VS VOUT VIN Figure 30. AD744 Connected as a Unity Gain Voltage Follower Table I. Recommended Values of CCOMP vs. Various Capacitive Loads Max –3 dB CLOAD CCOMP Slew Rate Bandwidth Gain (pF) (pF) (V/ s) (MHz) 1 50 5 37 6.5 1 150 10 25 4.3 1 2000 25 12.5 2.0 Figures 31 and 32 show the AD744 as a voltage follower with gain and as an inverting amplifier. In these cases, external compensation is not necessary for stable operation. How- ever, compensation may be applied to drive capacitive loads above 50 pF. Table II gives recommended CCOMP values, along with expected slew rates and bandwidths for a variety of load conditions and gains for the circuits in Figures 31 and 32. AD744 1 F 0.1 F –VS CCOMP 1 F 0.1 F +VS VOUT VIN OPTIONAL R2* CLEAD* R1* *SEE TABLE II Figure 31. AD744 Connected as a Voltage Follower Operating at Gains of 2 or Greater |
Ähnliche Teilenummer - AD744KR-REEL7 |
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Ähnliche Beschreibung - AD744KR-REEL7 |
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