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AD797AR-REEL データシート(PDF) 10 Page - Analog Devices |
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AD797AR-REEL データシート(HTML) 10 Page - Analog Devices |
10 / 16 page AD797 REV. C –10– BYPASSING CONSIDERATIONS To take full advantage of the very wide bandwidth and dynamic range capabilities of the AD797 requires some precautions. First, multiple bypassing is recommended in any precision application. A 1.0 µF–4.7 µF tantalum in parallel with 0.1 µF ceramic bypass capacitors are sufficient in most applications. When driving heavy loads a larger demand is placed on the sup- ply bypassing. In this case selective use of larger values of tanta- lum capacitors and damping of their lead inductance with small value (1.1 Ω to 4.7 Ω) carbon resistors can be an improvement. Figure 32 summarizes bypassing recommendations. The symbol (**) is used throughout this data sheet to represent the parallel combination of a 0.1 µF and a 4.7 µF capacitor. VS 0.1 µF USE SHORT LEAD LENGTHS (<5mm) KELVIN RETURN LOAD CURRENT 4.7 – 22.0 µF USE SHORT LEAD RETURNS (<5mm) OR 4.7 µF VS 1.1 – 4.7 Ω 0.1 µF KELVIN RETURN LOAD CURRENT Figure 32. Recommended Power Supply Bypassing THE NONINVERTING CONFIGURATION Ultralow noise requires very low values of rBB’ (the internal parasitic resistance) for the input transistors ( ≈6 Ω). This im- plies very little damping of input and output reactive interac- tions. With the AD797, additional input series damping is required for stability with direct input to output feedback. A 100 Ω resistor in the inverting input (Figure 33) is sufficient; the 100 Ω balancing resistor (R2) is recommended, but is not required for stability. The noise penalty is minimal (eNtotal ≈2.1 nV/√Hz), which is usually insignificant. Best response flatness is obtained with the addition of a small capacitor (CL < 33 pF) in parallel with the 100 Ω resistor (Figure 34). The input source resistance and capacitance will also affect the response slightly and experimentation may be necessary for best results. R2 100 Ω R1 100 Ω VOUT ** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32. VIN RL 600 Ω AD797 ** 2 7 3 4 6 ** –VS +VS Figure 33. Voltage Follower Connection Low noise preamplification is usually done in the noninverting mode (Figure 35). For lowest noise the equivalent resistance of the feedback network should be as low as possible. The 30 mA minimum drive current of the AD797 makes it easier to achieve this. The feedback resistors can be made as low as possible with due consideration to load drive and power consumption. Table II gives some representative values for the AD797 as a low noise follower. Operation on 5 volt supplies allows the use of a 100 Ω or less feedback network (R1 + R2). Since the AD797 shows no unusual behavior when operating near its maximum rated current, it is suitable for driving the AD600/AD602 (Figure 47) while preserving their low noise performance. Optimum flatness and stability at noise gains >1 sometimes requires a small capacitor (CL) connected across the feedback resistor (R1, Figure 35). Table II includes recommended values of CL for several gains. In general, when R2 is greater than 100 Ω and C L is greater than 33 pF, a 100 Ω resistor should be placed in series with CL. Source resistance matching is assumed, and the AD797 should never be operated with unbal- anced source resistance >200 k Ω/G. RS* CS* 100 AD797 ** 2 7 3 4 6 ** VOUT –VS +VS * SEE TEXT ** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32. VIN 600 CL Ω Ω Figure 34. Alternative Voltage Follower Connection R2 R1 R L AD797 ** 2 7 3 4 6 ** VOUT –VS +VS ** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32. VIN CL Figure 35. Low Noise Preamplifier Table II. Values for Follower With Gain Circuit Noise Gain R1 R2 CL (Excluding rS) 21 k Ω 1 k Ω≈20 pF 3.0 nV/ √Hz 2 300 Ω 300 Ω≈10 pF 1.8 nV/ √Hz 10 33.2 Ω 300 Ω≈5 pF 1.2 nV/ √Hz 20 16.5 Ω 316 Ω 1.0 nV/ √Hz >35 10 Ω (G–1) • 10 Ω 0.98 nV/ √Hz The I-to-V converter is a special case of the follower configura- tion. When the AD797 is used in an I-to-V converter, for in- stance as a DAC buffer, the circuit of Figure 36 should be used. The value of CL depends on the DAC and again, if CL is |
同様の部品番号 - AD797AR-REEL |
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同様の説明 - AD797AR-REEL |
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