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AD844JR Fiches technique(PDF) 9 Page - Analog Devices |
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AD844JR Fiches technique(HTML) 9 Page - Analog Devices |
9 / 12 page 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 28) is not designed to provide high input volt- age 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). Figure 20 shows a noninverting amplifier with a gain of 10 and a bandwidth of 30 MHz. The transient response is shown in Fig- ures 23 and 24. To increase the bandwidth at higher gains, a ca- pacitor can be added across R2 whose value is approximately the ratio of R1 and R2 times Ct. Figure 29. Noninverting Amplifier Gain = 100, Optional Offset Trim Is Shown Noninverting Gain of 100 The AD844 provides very clean pulse response at high nonin- verting gains. Figure 29 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 peaking capacitor (CPK) can optionally be added to further extend the bandwidth. Figure 30 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 29. Figure 30. AC Response for Gain = 100, Configuration Shown in Figure 29 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. Input Impedance At low frequencies, negative feedback keeps the resistance at the inverting input close to zero. As the frequency increases, the im- pedance looking into this input will increase from near zero to the open loop input resistance, due to bandwidth limitations, making the input seem inductive. If it is desired to keep the in- put impedance flatter, a series RC network can be inserted across the input. The resistor is chosen so that the parallel sum of it and R2 equals the desired termination resistance. The ca- pacitance is set so that the pole determined by this RC network is about half the bandwidth of the op amp. This network is not important if the input resistor is much larger than the termina- tion used, or if frequencies are relatively low. In some cases, the small peaking that occurs without the network can be of use in extending the –3 dB bandwidth. Applying the AD844 REV. C –9– |
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