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參數(shù)資料
| 型號(hào)： | AD8128ACPZ-R7 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 12/12頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC RECEIVER LINE EQUAL 8LFCSP |
| 標(biāo)準(zhǔn)包裝： | 1,500 |
| 類型： | 接收器 |
| 驅(qū)動(dòng)器/接收器數(shù)： | 0/1 |
| 規(guī)程： | 以太網(wǎng) |
| 電源電壓： | 4.5 V ~ 5.5 V |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 8-VFDFN 裸露焊盤，CSP |
| 供應(yīng)商設(shè)備封裝： | 8-LFCSP-VD（3x3） |
| 包裝： | 帶卷 (TR) |
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
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.
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