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AD8005ART-REEL датащи(PDF) 9 Page - Analog Devices |
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AD8005ART-REEL датащи(HTML) 9 Page - Analog Devices |
9 / 12 page AD8005 –9– REV. A APPLICATIONS Driving Capacitive Loads Capacitive loads interact with an op amp’s output impedance to create an extra delay in the feedback path. This reduces circuit stability, and can cause unwanted ringing and oscilla- tion. A given value of capacitance causes much less ringing when the amplifier is used with a higher noise gain. The capacitive load drive of the AD8005 can be increased by adding a low valued resistor in series with the capacitive load. Introducing a series resistor tends to isolate the capacitive load from the feedback loop thereby diminishing its influence. Fig- ure 29 shows the effects of a series resistor on capacitive drive for varying voltage gains. As the closed-loop gain is increased, the larger phase margin allows for larger capacitive loads with less overshoot. Adding a series resistor at lower closed-loop gains accomplishes the same effect. For large capacitive loads, the frequency response of the amplifier will be dominated by the roll-off of the series resistor and capacitive load. AD8005 RG RF RS RL 1k CL Figure 28. Driving Capacitive Loads 80 70 60 1 345 CLOSED-LOOP GAIN – V/V 50 2 40 30 20 10 0 VS = 5V RS = 10 RS = 5 RS = 0 2V OUTPUT STEP WITH 30% OVERSHOOT Figure 29. Capacitive Load Drive vs. Closed-Loop Gain Single-Supply Level Shifter In addition to providing buffering, many systems require that an op amp provide level shifting. A common example is the level shifting that is required to move a bipolar signal into the unipo- lar range of many modern analog-to-digital converters (ADCs). In general, single supply ADCs have input ranges that are refer- enced neither to ground nor supply. Instead the reference level is some point in between, usually halfway between ground and supply (+2.5 V for a single supply 5 V ADC). Because high- speed ADCs typically have input voltage ranges of 1 V to 2 V, the op amp driving it must be single supply but not necessarily rail-to-rail. VREF +5V R2 1.5k VOUT 0.01 F 10 F +5V R1 1.5k VIN 0.1 F R4 10k R3 30.1k AD8005 Figure 30. Bipolar to Unipolar Level Shifter Figure 30 shows a level shifter circuit that can move a bipolar signal into a unipolar range. A positive reference voltage, derived from the +5 V supply, sets a bias level of +1.25 V at the nonin- verting terminal of the op amp. In ac applications, the accuracy of this voltage level is not important. Noise is however a serious consideration. A 0.1 µF capacitor provides useful decoupling of this noise. The bias level on the noninverting terminal sets the input common- mode voltage to +1.25 V. Because the output will always be positive, the op amp may therefore be powered with a single +5 V power supply. The overall gain function is given by the equation: VOUT = – R2 R1 V IN + R4 R3 + R4 1 + R2 R1 V REF In the above example, the equation simplifies to VOUT = –V IN + 2.5V |
Аналогичный номер детали - AD8005ART-REEL |
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Аналогичное описание - AD8005ART-REEL |
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