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
V
GAIN
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 V
PEAK
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 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
fjkl
efH
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.
V
ICM
= V
CM
+ V
SYNC
+ V
OTHER
(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,
V
CM
= 2.5 V. Furthermore, an addition of a sync signal, V
SYNC
=
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.
