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ISL1557IUEZ-T7 Fiches technique(PDF) 8 Page - Renesas Technology Corp |
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ISL1557IUEZ-T7 Fiches technique(HTML) 8 Page - Renesas Technology Corp |
8 / 12 page ISL1557 FN7522 Rev 3.00 Page 8 of 12 Apr 6, 2018 Applications Information Product Description The ISL1557 is a dual operational amplifier designed for line driving in DMT ADSL2+ and VDSL solutions. It is a dual current mode feedback amplifier with low distortion while drawing moderately low supply current. It is built using the Renesas proprietary complimentary bipolar process and is offered in industry standard pinouts. Due to the current feedback architecture, the ISL1557 closed-loop 3dB bandwidth is dependent on the value of the feedback resistor. First, select the desired bandwidth by choosing the feedback resistor, RF, then set the gain by choosing the gain resistor, RG. The curves at the beginning of the “Typical Performance Curves” section, on page 4, show the effect of varying both RF and RG. The 3dB bandwidth is somewhat dependent on the power supply voltage. Power Supply Bypassing and Printed Circuit Board Layout As with any high frequency device, good printed circuit board layout is necessary for optimum performance. Ground plane construction is highly recommended. Lead lengths should be as short as possible (below 0.25”). The power supply pins must be well bypassed to reduce the risk of oscillation. A 4.7µF tantalum capacitor in parallel with a 0.1µF ceramic capacitor is adequate for each supply pin. During power-up, it is necessary to limit the slew rate of the rising power supply to within 1V/µs. If the power supply rising time is undetermined, a series 10Ω resistor on the power supply line can be used to ensure the proper power supply rise time. For good AC performance, parasitic capacitances should be kept to a minimum, especially at the inverting input. This implies keeping the ground plane away from this pin. Carbon resistors are acceptable, but avoid using wire-wound resistors because of their parasitic inductance. Similarly, capacitors should be low inductance for best performance. Capacitance at the Inverting Input Due to the topology of the current feedback amplifier, stray capacitance at the inverting input will effect the AC and transient performance of the ISL1557 when operating in the non-inverting configuration. In the inverting gain mode, added capacitance at the inverting input has little effect because this point is at a virtual ground and stray capacitance is therefore not “detected” by the amplifier. Feedback Resistor Values The ISL1557 has been designed and specified with RF = 750Ω for AV = +5. This value of feedback resistor yields extremely flat frequency response with 1dB peaking out to 250MHz. As with all current feedback amplifiers, wider bandwidth, at the expense of slight peaking, can be obtained by reducing the value of the feedback resistor. Inversely, larger values of feedback resistor will cause rolloff to occur at a lower frequency. See the curves in the “Typical Performance Curves” section, beginning on page 4, which shows 3dB bandwidth and peaking vs frequency for various feedback resistors and various supply voltages. Bandwidth vs Temperature Whereas many amplifier's supply current and consequently 3dB bandwidth drop off at high temperature, the ISL1557 is designed to have little supply current variations with temperature. An immediate benefit is the 3dB bandwidth does not drop off drastically with temperature. Supply Voltage Range The ISL1557IRZ is designed to operate with supply voltages from ±2.25V to ±6V nominal. Optimum bandwidth, slew rate, and video characteristics are obtained at higher supply voltages. Single Supply Operation If a single supply is desired, values from +4.5V to +12V nominal can be used as long as the input common mode range is not exceeded. When using a single supply, be sure to either: • DC bias the inputs at an appropriate common mode voltage and AC couple the signal, or: • Ensure the driving signal is within the common mode range of the ISL1557. The ISL1557IUEZ must be used in single supply applications. ADSL CPE Applications The ISL1557 is designed as a line driver for ADSL CPE modems. It is capable of outputting 450mA of output current with a typical supply voltage headroom of 1.3V. It can achieve -85dBc of distortion at low 7.1mA of supply current per amplifier. The average line power requirement for the ADSL CPE application is 14.5dBm (28mW) into a 100Ω line. The average line voltage is 1.67VRMS. The ADSL DMT peak to average ratio (crest factor) of 5.3 implies peak voltage of 7.5V into the line. Using a differential drive configuration and transformer coupling with standard back termination, a transformer ratio of 1:2 is selected. The circuit configuration is shown in Figure 24. - + - + TX1 1:2 12.5 12.5 750 750 250Ω AFE 100 FIGURE 24. CIRCUIT CONFIGURATION |
Numéro de pièce similaire - ISL1557IUEZ-T7 |
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Description similaire - ISL1557IUEZ-T7 |
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