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AD8883B 데이터시트(PDF) 9 Page - Analog Devices |
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AD8883B 데이터시트(HTML) 9 Page - Analog Devices |
9 / 16 page REV. D AD844 –9– It is important to understand that the low input impedance at the inverting input is locally generated, and does not depend on feedback. This is very different from the “virtual ground” of a conventional operational amplifier used in the current summing mode which is essentially an open circuit until the loop settles. In the AD844, transient current at the input does not cause voltage spikes at the summing node while the amplifier is settling. Furthermore, all of the transient current is delivered to the slewing (TZ) node (Pin 5) via a short signal path (the grounded base stages and the wideband current mirrors). The current available to charge the capacitance (about 4.5 pF) at TZ node, is always proportional to the input error current, and the slew rate limitations associated with the large signal response of op amps do not occur. For this reason, the rise and fall times are almost independent of signal level. In practice, the input current will eventually cause the mirrors to saturate. When using ± 15 V supplies, this occurs at about 10 mA (or ± 2200 V/µs). Since signal currents are rarely this large, classical “slew rate” limitations are absent. This inherent advantage would be lost if the voltage follower used to buffer the output were to have slew rate limitations. The AD844 has been designed to avoid this problem, and as a result the output buffer exhibits a clean large signal transient response, free from anomalous effects arising from internal saturation. Response as a Noninverting Amplifier Since current feedback amplifiers are asymmetrical with regard to their two inputs, performance will differ markedly in nonin- verting and inverting modes. In noninverting modes, the large signal high speed behavior of the AD844 deteriorates at low gains because the biasing circuitry for the input system (not shown in Figure 4) is not designed to provide high input voltage slew rates. However, good results can be obtained with some care. The noninverting input will not tolerate a large transient input; it must be kept below ±1 V for best results. Consequently this mode is better suited to high gain applications (greater than ×10). TPC 20 shows a noninverting amplifier with a gain of 10 and a bandwidth of 30 MHz. The transient response is shown in TPCs 23 and 24. To increase the bandwidth at higher gains, a capacitor can be added across R2 whose value is approximately the ratio of R1 and R2 times Ct. Noninverting Gain of 100 The AD844 provides very clean pulse response at high nonin- verting gains. Figure 5 shows a typical configuration providing a gain of 100 with high input resistance. The feedback resistor is kept as low as practicable to maximize bandwidth, and a peak- ing capacitor (CPK) can optionally be added to further extend the bandwidth. Figure 6 shows the small signal response with CPK = 3 nF, RL = 500 Ω, and supply voltages of either ±5 V or ±15 V. Gain bandwidth products of up to 900 MHz can be achieved in this way. The offset voltage of the AD844 is laser trimmed to the 50 µV level and exhibits very low drift. In practice, there is an addi- tional offset term due to the bias current at the inverting input (IBN) which flows in the feedback resistor (R1). This can option- ally be nulled by the trimming potentiometer shown in Figure 5. 8 OFFSET TRIM CPK 3nF 20 4.99 4.7 499 0.22 F 0.22 F RL VIN +VS –VS AD844 R1 R2 4.7 + – Figure 5. Noninverting Amplifier Gain = 100, Optional Offset Trim Is Shown FREQUENCY – Hz 46 16 100k 1M 10M 40 34 28 22 20M VS = 15V VS = 5V Figure 6. AC Response for Gain = 100, Configuration Shown in Figure 5 USING THE AD844 Board Layout As with all high frequency circuits considerable care must be used in the layout of the components surrounding the AD844. A ground plane, to which the power supply decoupling capaci- tors are connected by the shortest possible leads, is essential to achieving clean pulse response. Even a continuous ground plane will exhibit finite voltage drops between points on the plane, and this must be kept in mind in selecting the grounding points. Generally speaking, decoupling capacitors should be taken to a point close to the load (or output connector) since the load currents flow in these capacitors at high frequencies. The +IN and –IN circuits (for example, a termination resistor and Pin 3) must be taken to a common point on the ground plane close to the amplifier package. Use low impedance capacitors (AVX SR305C224KAA or equivalent) of 0.22 µF wherever ac coupling is required. Include either ferrite beads and/or a small series resistance (approxi- mately 4.7 Ω) in each supply line. |
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