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5962F9683101VPA Datasheet(PDF) 2 Page - Intersil Corporation |
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5962F9683101VPA Datasheet(HTML) 2 Page - Intersil Corporation |
2 / 9 page 2 Application Information HS-1212RH Advantages The HS-1212RH features a novel design which allows the user to select from three closed loop gains, without any external components. The result is a more flexible product, fewer part types in inventory, and more efficient use of board space. Implementing a dual, gain of 2, cable driver with this IC eliminates the four gain setting resistors, which frees up board space for termination resistors. Like most newer high performance amplifiers, the HS-1212RH is a current feedback amplifier (CFA). CFAs offer high bandwidth and slew rate at low supply currents, but can be difficult to use because of their sensitivity to feedback capacitance and parasitics on the inverting input (summing node). The HS-1212RH eliminates these concerns by bringing the gain setting resistors on-chip. This yields the optimum placement and value of the feedback resistor, while minimizing feedback and summing node parasitics. Because there is no access to the summing node, the PCB parasitics do not impact performance at gains of +2 or -1 (see “Unity Gain Considerations” for discussion of parasitic impact on unity gain performance). The HS-1212RH’s closed loop gain implementation provides better gain accuracy, lower offset and output impedance, and better distortion compared with open loop buffers. Closed Loop Gain Selection This “buffer” operates in closed loop gains of -1, +1, or +2, with gain selection accomplished via connections to the inputs. Applying the input signal to +IN and floating -IN selects a gain of +1 (see next section for layout caveats), while grounding -IN selects a gain of +2. A gain of -1 is obtained by applying the input signal to -IN with +IN grounded through a 50 Ω resistor. The table below summarizes these connections: Unity Gain Considerations Unity gain selection is accomplished by floating the -Input of the HS-1212RH. Anything that tends to short the -Input to GND, such as stray capacitance at high frequencies, will cause the amplifier gain to increase toward a gain of +2. The result is excessive high frequency peaking, and possible instability. Even the minimal amount of capacitance associated with attaching the -Input lead to the PCB results in approximately 6dB of gain peaking. At a minimum this requires due care to ensure the minimum capacitance at the -Input connection. Table 1 lists five alternate methods for configuring the HS-1212RH as a unity gain buffer, and the corresponding performance. The implementations vary in complexity and involve performance trade-offs. The easiest approach to implement is simply shorting the two input pins together, and applying the input signal to this common node. The amplifier bandwidth decreases from 430MHz to 280MHz, but excellent gain flatness is the benefit. A drawback to this approach is that the amplifier input noise voltage and input offset voltage terms see a gain of +2, resulting in higher noise and output offset voltages. Alternately, a 100pF capacitor between the inputs shorts them only at high frequencies, which prevents the increased output offset voltage but delivers less gain flatness. Another straightforward approach is to add a 620 Ω resistor in series with the amplifier’s positive input. This resistor and the HS-1212RH input capacitance form a low pass filter which rolls off the signal bandwidth before gain peaking occurs. This configuration was employed to obtain the data sheet AC and transient parameters for a gain of +1. Pulse Overshoot The HS-1212RH 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 overshoot on the negative portion of the output waveform (see Figure 6, Figure 9, and Figure 12). This overshoot isn’t present for small bipolar signals (see Figure 4, Figure 7, and Figure 10) or large positive signals (see Figure 5, Figure 8 and Figure 11). PC Board Layout This amplifier’s frequency response depends greatly on the care taken in designing the PC board (PCB). 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. GAIN (ACL) CONNECTIONS +INPUT -INPUT -1 50 Ω to GND Input +1 Input NC (Floating) +2 Input GND HS-1212RH |
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