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AD8153 Datasheet(PDF) 20 Page - Analog Devices

Part No. AD8153
Description  Single Buffered Mux/Demux Switch
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

AD8153 Datasheet(HTML) 20 Page - Analog Devices

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AD8153
Rev. 0 | Page 20 of 24
INTERFACING TO THE AD8153
TERMINATION STRUCTURES
To determine the best strategy for connecting to the high speed
pins of the AD8153, the user must first be familiar with the on-
chip termination structures. The AD8153 contains two types of
these structures: one type for input ports and one type for output
ports (see Figure 37 and Figure 38).
VCC
VTTI
IPX
INX
VEE
55Ω
55Ω
1173Ω
Figure 37. Receiver Simplified Diagram
VCC
VTTO
OPX
ONX
VEE
50Ω
50Ω
VIP
VIN
IT
Figure 38. Transmitter Simplified Diagram
For input ports, the termination structure consists of two 55 Ω
resistors connected to a termination supply and an 1173 Ω
resistor connected across the differential inputs, the latter being
a result of the finite differential input impedance of the equalizer.
For output ports, there are two 50 Ω resistors connected to the
termination supply. Note that the differential input resistance
for both structures is the same, 100 Ω.
INPUT COMPLIANCE
The range of allowable input voltages is determined by the
fundamental limitations of the active input circuitry. This range
of signals is normally a function of the common-mode level of
the input signal, the signal swing, and the supply voltage. For a
given input signal swing, there is a range of common-mode
voltages that keeps the high and low voltage excursions within
acceptable limits. Similarly, for a given common-mode input
voltage, there is a maximum acceptable input signal swing.
There is also a minimum signal swing that the active input
circuitry can resolve reliably. The specifications are found in
Table 1.
AC Coupling
One way to simplify the input circuit and make it compatible
with a wide variety of driving devices is to use ac coupling. This
has the effect of isolating the dc common-mode levels of the
driver and the AD8153 input circuitry. AC coupling requires a
capacitor in series with each single-ended input signal, as shown
in Figure 39. This should be done in a manner that does not
interfere with the high speed signal integrity of the PCB.
50Ω
50Ω
VCC
VCC
55Ω
VTTI
DRIVER
AD8153
55Ω
VEE
1173Ω
CP
CN
IP
IN
Figure 39. AC-Coupling Input Signal of AD8153
When ac coupling is used, the common-mode level at the input
of the device is equal to VTTI. The single-ended input signal
swings above and below VTTI equally. The user can then use
the specifications in Table 1 to determine the input signal swing
levels that satisfy the input range of the AD8153.
If dc coupling is required, determining the input common-
mode level is less straightforward because the configuration of
the driver must also be considered. In most cases, the user
would set VTTI on the AD8153 to the same level as the driver
output termination voltage. This prevents a continuous dc
current from flowing between the two supply nets. As a
practical matter, both devices can be terminated to the same
physical supply net.
Consider the following example: a driver is dc-coupled to the
input of the AD8153. The AD8153 input termination voltage
(VTTI) and the driver output termination voltage (VTTOD) are both
set to the same level; that is, VTTI = VTTOD = 3.3 V. If an 800 mV
differential p-p swing is desired, the total output current of the
driver is 16 mA. At balance, the output current is divided evenly
between the two sides of the differential signal path, 8 mA to each
side. This 8 mA of current flows through the parallel combina-
tion of the 55 Ω input termination resistor on the AD8153 and
the 50 Ω output termination resistor on the driver, resulting in a
common-mode level of
VTTI − 8 mA × (50 Ω || 55 Ω) = VTTI − 209 mV
The user can then determine the allowable range of values for VTTI
that meets the input compliance range based on an 800 mV p-p
differential swing.


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