Datasheet AD8128 (Analog Devices) - 9

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Data Sheet AD8128 THEORY OF OPERATION
The AD8128 is a high speed, low noise analog line equalizer The AD8128 approximates the magnitude response of that compensates for losses in CAT-5/CAT-5e cables up to Equation 1 by summing multiple zero-poles pairs offset at 100 meters with ±1 dB flatness in the pass band out to 70 MHz different frequencies. Equalization adjustment due to varying (see Figure 8). Two continuously adjustable control voltages line lengths is done by changing the weighting factors of each of alter the frequency response to add flexibility to the system by the zero-pole pairs. allowing for the compensation of various cable lengths as well
INPUT COMMON-MODE VOLTAGE RANGE
as for variations in the cable itself. The dc control voltage pin
CONSIDERATIONS
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 When using the AD8128 as a receiver, it is important to ensure control voltage pin V that the input common-mode (CM) voltage range of the AD8128 PEAK adjusts the amount of high frequency peaking (see Figure 5) from 0 dB to 20 dB. This compensates stays within the specified range. The input CM level can be easily for the high frequency loss due to the skin effect of the cable. calculated by adding the CM level of the driver, the amplitude of any sync pulses, and the other possible induced common-mode The AD8128 has a high impedance differential input that allows signals from power lines and fluorescent lights. it to receive dc-coupled signals directly from the cable. For systems with very high CMRR specifications, the AD8128 can VICM = VCM + VSYNC + VOTHER (2) also be used with a dedicated receiver, such as the AD8130 or For example, when using a single 5 V supply on the drive side, AD8143, placed in front of it. The output of the AD8128 is low the CM voltage of the line typically becomes the midsupply voltage, impedance and is capable of driving a 150 Ω load resistor and VCM = 2.5 V. Furthermore, an addition of a sync signal, VSYNC = up to 20 pF of load capacitance at the output. For systems with 0.5 V, on to the common mode puts the peak CM voltage at 3 V. high parasitic capacitances at the output, it is recommended Assuming that both the driver and receiver have exactly the same that a small series resistor be placed between the output and ground potential, the signal is marginally below the upper end capacitive load to reduce ringing in the pulse response. of the common-mode input range of 3.1 V. Other CM signals The AD8128 is designed to be used in medium-length systems that can be picked up by the CAT-5 cable result in exceeding the that have stringent low noise requirements as well as longer- CM input range of the AD8128. length systems that can tolerate more noise. For the medium- The most effective way of not exceeding the CM level of the length requirements, a single AD8128 is able to compensate up AD8128 is to lower the CM level on the driver. In the previous to 100 meters of cable with only 1.5 mV rms of output noise. example, this was the primary contributor to the CM input level. For longer-length applications that require equalization of up to If this is not possible, a dedicated receiver with a wider CM input 200 meters of cable, two AD8128 devices can be cascaded together range, such as the AD8130 or AD8143, must be used. 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 H f  kl  1 j  f  e (1) where: f is the frequency. l is the length. k is the line constant. Rev. A | Page 9 of 12 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE Maximum Power Dissipation ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS TEST CIRCUIT THEORY OF OPERATION INPUT COMMON-MODE VOLTAGE RANGE CONSIDERATIONS APPLICATIONS INFORMATION KVM APPLICATIONS DC CONTROL PINS CASCADED APPLICATIONS EXPOSED PAD (EP) LAYOUT AND POWER SUPPLY DECOUPLING CONSIDERATIONS EVALUATION BOARDS OUTLINE DIMENSIONS ORDERING GUIDE