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50205Z Datasheet(PDF) 10 Page - Intersil Corporation |
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50205Z Datasheet(HTML) 10 Page - Intersil Corporation |
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10 / 23 page  10 FN2845.11 June 5, 2006 Application Information Optimum Feedback Resistor The plots of inverting and non-inverting frequency response illustrate the performance of the HA-5020 in various closed loop gain configurations. Although the bandwidth dependency on closed loop gain isn’t as severe as that of a voltage feedback amplifier, there can be an appreciable decrease in bandwidth at higher gains. This decrease may be minimized by taking advantage of the current feedback amplifier’s unique relationship between bandwidth and RF. All current feedback amplifiers require a feedback resistor, even for unity gain applications, and RF, in conjunction with the internal compensation capacitor, sets the dominant pole of the frequency response. Thus, the amplifier’s bandwidth is inversely proportional to RF. The HA-5020 design is optimized for a 1000 Ω R F at a gain of +1. Decreasing RF in a unity gain application decreases stability, resulting in excessive peaking and overshoot. At higher gains the amplifier is more stable, so RF can be decreased in a trade-off of stability for bandwidth. The table below lists recommended RF values for various gains, and the expected bandwidth. 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. If leaded components are used the leads must be kept short especially for the power supply decoupling components and those components connected to the inverting input. Attention must be given to decoupling the power supplies. A large value (10 µF) tantalum or electrolytic capacitor in parallel with a small value (0.1 µF) chip capacitor works well in most cases. A ground plane is strongly recommended to control noise. Care must also be taken to minimize the capacitance to ground seen by the amplifier’s inverting input (-IN). The larger this capacitance, the worse the gain peaking, resulting in pulse overshoot and possible instability. It is recommended that the ground plane be removed under traces connected to -IN, and that connections to -IN be kept as short as possible to minimize the capacitance from this node to ground. Driving Capacitive Loads Capacitive loads 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 an isolation resistor (R) in series with the output as shown in Figure 6. The selection criteria for the isolation resistor is highly dependent on the load, but 27 Ω has been determined to be a good starting value. Enable/Disable Function When enabled the amplifier functions as a normal current feedback amplifier with all of the data in the electrical specifications table being valid and applicable. When disabled the amplifier output assumes a true high impedance state and the supply current is reduced significantly. The circuit shown in Figure 7 is a simplified schematic of the enable/disable function. The large value resistors in series with the DISABLE pin makes it appear as a current source to the driver. When the driver pulls this pin low current flows out of the pin and into the driver. This current, which may be as large as 350 µA when external circuit and process variables are at their extremes, is required to insure that point “A” achieves the proper potential to disable the output. The driver must have the compliance and capability of sinking all of this current. When VCC is +5V the DISABLE pin may be driven with a dedicated TTL gate. The maximum low level output voltage of the TTL gate, 0.4V, has enough compliance to insure that the amplifier will always be disabled even though D1 will not turn on, and the TTL gate will sink enough current to keep point “A” at its proper voltage. When VCC is greater than +5V the DISABLE pin should be driven with an open collector device that has a breakdown rating greater than VCC. GAIN (ACL)RF (Ω) BANDWIDTH (MHz) -1 750 100 +1 1000 125 +2 681 95 +5 1000 52 +10 383 65 -10 750 22 VIN VOUT CL RT +- RI RF R FIGURE 6. PLACEMENT OF THE OUTPUT ISOLATION RESISTOR, R R6 15K R7 15K +VCC ENABLE/ D1 QP3 R8 QP18 A R33 R10 DISABLE INPUT FIGURE 7. SIMPLIFIED SCHEMATIC OF ENABLE/DISABLE FUNCTION HA-5020 |
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