Data Sheet

AD8128

Rev. A | 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

account for dc resistive losses present in the cable. A second dc

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 the 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 AD8128 devices 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



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.

(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,

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, must be used.