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HFA1205 Datasheet(PDF) 5 Page - Intersil Corporation |
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HFA1205 Datasheet(HTML) 5 Page - Intersil Corporation |
5 / 15 page 5 Non-inverting Input Source Impedance For best operation, the DC source impedance seen by the non-inverting input should be ≥50Ω. This is especially important in inverting gain configurations where the non- inverting input would normally be connected directly to GND. Pulse Undershoot and Asymmetrical Slew Rates The HFA1205 utilizes a quasi-complementary output stage to achieve high output current while minimizing quiescent supply current. In this approach, a composite device replaces the traditional PNP pulldown transistor. The composite device switches modes after crossing 0V, resulting in added distortion for signals swinging below ground, and an increased undershoot on the negative portion of the output waveform (see Figures 7, 11, 15 and 19). This undershoot isn’t present for small bipolar signals, or large positive signals (see Figures 5, 6, 9, 10, 13, 14, 17 and 18). Another artifact of the composite device is asymmetrical slew rates for output signals with a negative voltage component. The slew rate degrades as the output signal crosses through 0V (see Figures 7, 11, 15, and 19), resulting in a slower overall negative slew rate. Positive only signals have symmetrical slew rates as illustrated in the large signal positive pulse response graphs (see Figures 5, 9, 13 and 17). PC Board Layout The frequency response of this amplifier depends greatly on the amount of care taken in designing the PC board. The use of low inductance components such as chip resistors and chip capacitors is strongly recommended, while a solid ground plane is a must! Attention should be given to decoupling the power supplies. A large value (10 µF) tantalum in parallel with a small value (0.1 µF) chip capacitor works well in most cases. Terminated microstrip signal lines are recommended at the input and output of the device. Capacitance directly on the output must be minimized, or isolated as discussed in the next section. Care must also be taken to minimize the capacitance to ground at the amplifier’s inverting input (-IN). The larger this capacitance, the worse the gain peaking, resulting in pulse overshoot and possible instability. Reduce this capacitance by removing the ground plane under traces connected to - IN, and keep connections to -IN as short as possible. Driving Capacitive Loads Capacitive loads, such as an A/D input, or an improperly terminated transmission line will degrade the amplifier’s phase margin resulting in frequency response peaking and possible oscillations. In most cases, the oscillation can be avoided by placing a resistor (RS) in series with the output prior to the capacitance. Figure 1 details starting points for the selection of this resistor. The points on the curve indicate the RS and CL combinations for the optimum bandwidth, stability, and settling time, but experimental fine tuning is recommended. Picking a point above or to the right of the curve yields an overdamped response, while points below or left of the curve indicate areas of underdamped performance. RS and CL form a low pass network at the output, thus limiting system bandwidth well below the amplifier bandwidth of 425MHz (for AV = +2). By decreasing RS as CL increases (as illustrated in the curves), the maximum bandwidth is obtained without sacrificing stability. In spite of this, bandwidth still decreases as the load capacitance increases. For example, at AV =+2, RS =50Ω, CL =22pF, the overall bandwidth is limited to 230MHz, and bandwidth drops to 80MHz at AV = +2, RS = 7Ω, CL = 390pF. Evaluation Board The performance of the HFA1205 may be evaluated using the HA5023 Evaluation Board. The performance of the HFA1205IB (SOIC) may be evaluated using the HA5023 Evaluation Board and a SOIC to DIP adaptor like the Aries Electronics Part Number 08-350000-10. The schematic for amplifier 1 and the board layout are shown in Figure 2 and Figure 3. Resistors RF, RG and RS may require a change to the appropriate value (see “Optimum Feedback Resistor” section) for the gain being evaluated. To order evaluation boards (Part Number HA5023EVAL), please contact your local sales office. 0 100 200 300 400 0 10 20 30 40 50 LOAD CAPACITANCE (pF) AV = +2 150 250 350 50 FIGURE 1. RECOMMENDED SERIES OUTPUT RESISTOR vs LOAD CAPACITANCE HFA1205 |
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