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AD8128 Folha de dados(PDF) 9 Page - Analog Devices |
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AD8128 Folha de dados(HTML) 9 Page - Analog Devices |
9 / 12 page AD8128 Rev. 0 | Page 9 of 12 THEORY OF OPERATION The AD8128 is a high speed, low noise analog line equalizer that compensates for losses in CAT-5/CAT-5e cables up to 100 meters with ±1 dB flatness in the pass band out to 70 MHz (see Figure 8). Two continuously adjustable control voltages alter the frequency response to add flexibility to the system by allowing for the compensation of various cable lengths as well as for variations in the cable itself. The dc control voltage pin VGAIN adjusts ac broadband gain from 0 dB to 3 dB (see Figure 6) to account for dc resistive losses present in the cable. A second dc control voltage pin VPEAK adjusts the amount of high frequency peaking (see Figure 5) from 0 dB to 20 dB. This compensates for the high frequency loss due to the skin effect of the cable. The AD8128 has a high impedance differential input that allows it to receive dc-coupled signals directly from the cable. For systems with very high CMRR specifications, the AD8128 can also be used with a dedicated receiver, such as the AD8130 or AD8143, placed in front of it. The output of the AD8128 is low impedance and is capable of driving a 150 Ω load resistor and up to 20 pF of load capacitance at its output. For systems with high parasitic capacitances at the output, it is recommended that a small series resistor be placed between the output and capacitive load to reduce ringing in the pulse response. The AD8128 is designed to be used in medium-length systems that have stringent low noise requirements as well as longer- length systems that can tolerate more noise. For the medium- length requirements, a single AD8128 is able to compensate up to 100 meters of cable with only 1.5 mV rms of output noise. For longer-length applications that require equalization of up to 200 meters of cable, two AD8128s can be cascaded together to achieve the desired equalization, while keeping approximately the same pass-band bandwidth, but with a slight degradation in settling time and slew rate. The frequency response of the AD8128 approximates the inverse frequency response of a lossy transmission line, which is given by () () f j kl e f H + = 1 (1) where: f is the frequency. l is the length. k is the line constant. The AD8128 approximates the magnitude response of Equation 1 by summing multiple zero-poles pairs offset at different frequencies. Equalization adjustment due to varying line lengths is done by changing the weighting factors of each of the zero-pole pairs. INPUT COMMON-MODE VOLTAGE RANGE CONSIDERATIONS When using the AD8128 as a receiver, it is important to ensure that the input common-mode (CM) voltage range of the AD8128 stays within the specified range. The input CM level can be easily calculated by adding the CM level of the driver, the amplitude of any sync pulses, and the other possible induced common-mode signals from power lines and fluorescent lights. VICM = VCM + VSYNC + VOTHER (2) For example, when using a single 5 V supply on the drive side, the CM voltage of the line typically becomes the midsupply voltage, VCM = 2.5 V. Furthermore, an addition of a sync signal, VSYNC = 0.5 V, on to the common mode puts the peak CM voltage at 3 V. Assuming that both the driver and receiver have exactly the same ground potential, the signal is marginally below the upper end of the common-mode input range of 3.1 V. Other CM signals that can be picked up by the CAT-5 cable result in exceeding the CM input range of the AD8128. The most effective way of not exceeding the CM level of the AD8128 is to lower the CM level on the driver. In the previous example, this was the primary contributor to the CM input level. If this is not possible, a dedicated receiver with a wider CM input range, such as the AD8130 or AD8143, should be used. |
Nº de peça semelhante - AD8128 |
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Descrição semelhante - AD8128 |
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