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EL5397CU Datasheet(PDF) 11 Page - Elantec Semiconductor |
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EL5397CU Datasheet(HTML) 11 Page - Elantec Semiconductor |
11 / 14 page 11 EL5397C - Preliminary Triple 200MHz Fixed Gain Amplifier Applications Information Product Description The EL5397C is a current-feedback operational ampli- fier that offers a wide -3dB bandwidth of 300MHz and a low supply current of 4mA per amplifier. The EL5397C works with supply voltages ranging from a single 5V to 10V and they are also capable of swinging to within 1V of either supply on the output. Because of their current- feedback topology, the EL5397C does not have the nor- mal gain-bandwidth product associated with voltage- feedback operational amplifiers. Instead, its -3dB band- width to remain relatively constant as closed-loop gain is increased. This combination of high bandwidth and low power, together with aggressive pricing make the EL5397C the ideal choice for many low-power/high- bandwidth applications such as portable, handheld, or battery-powered equipment. For varying bandwidth needs, consider the EL5191C with 1GHz on a 9mA supply current or the EL5192C with 600MHz on a 6mA supply current. Versions include single, dual, and triple amp packages with 5-pin SOT23, 16-pin QSOP, and 8-pin or 16-pin SO outlines. Power Supply Bypassing and Printed Circuit Board Layout As with any high frequency device, good printed circuit board layout is necessary for optimum performance. Low impedance ground plane construction is essential. Surface mount components are recommended, but if leaded components are used, lead lengths should be as short as possible. The power supply pins must be well bypassed to reduce the risk of oscillation. The combina- tion of a 4.7µF tantalum capacitor in parallel with a 0.01µF capacitor has been shown to work well when placed at each supply pin. For good AC performance, parasitic capacitance should be kept to a minimum, especially at the inverting input. (See the Capacitance at the Inverting Input section) Even when ground plane construction is used, it should be removed from the area near the inverting input to mini- mize any stray capacitance at that node. Carbon or Metal-Film resistors are acceptable with the Metal-Film resistors giving slightly less peaking and bandwidth because of additional series inductance. Use of sockets, particularly for the SO package, should be avoided if possible. Sockets add parasitic inductance and capaci- tance which will result in additional peaking and overshoot. Capacitance at the Inverting Input Any manufacturer’s high-speed voltage- or current- feedback amplifier can be affected by stray capacitance at the inverting input. For inverting gains, this parasitic capacitance has little effect because the inverting input is a virtual ground, but for non-inverting gains, this capac- itance (in conjunction with the feedback and gain resistors) creates a pole in the feedback path of the amplifier. This pole, if low enough in frequency, has the same destabilizing effect as a zero in the forward open- loop response. The use of large-value feedback and gain resistors exacerbates the problem by further lowering the pole frequency (increasing the possibility of oscillation.) The EL5397C has been optimized with a 475 Ω feedback resistor. With the high bandwidth of these amplifiers, these resistor values might cause stability problems when combined with parasitic capacitance, thus ground plane is not recommended around the inverting input pin of the amplifier. Feedback Resistor Values The EL5397C has been designed and specified at a gain of +2 with RF approximately 500Ω. This value of feed- back resistor gives 200MHz of -3dB bandwidth at AV=2 with 2dB of peaking. With AV=-2, an RF of approxi- mately 500 Ω gives 175MHz of bandwidth with 0.2dB of peaking. Since the EL5397C is a current-feedback amplifier, it is also possible to change the value of RF to get more bandwidth. As seen in the curve of Frequency Response for Various RF and RG, bandwidth and peak- ing can be easily modified by varying the value of the feedback resistor. Because the EL5397C is a current-feedback amplifier, its gain-bandwidth product is not a constant for different closed-loop gains. This feature actually allows the EL5397C to maintain about the same -3dB bandwidth. As gain is increased, bandwidth decreases slightly while |
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