AD8158 查看數據表（PDF） - Analog Devices
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AD8158 Datasheet PDF : 36 Pages
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AD8158
The first section is the input termination resistors. The power
dissipated in the termination resistors is due to the input
differential swing and any common-mode current resulting
from dc-coupling the input.
In the next section, the receiver, each input is powered only
when it is selected and the disable bits are set to 0. If a receiver
is not selected, it is powered down. Thus, the total number of
active inputs affects the total power consumption. Furthermore,
the loss-of-signal detection circuits can be disabled independent
of the receiver for even greater power savings.
The core of the device performs the multiplexer and demulti-
plexer switching functions. It draws a fixed quiescent current of
2 mA whenever the AD8158 is powered from VCC to VEE. The
switch draws an additional 8 × 4.6 mA in normal mux/demux
operation and an additional 12 × 4.6 mA with all ports in
loopback or with bicast selected. The switch core can be
disabled to save power.
An output predriver section draws a current, IPRED, that is
related to the programmed output current, ITOT. Table 20 lists
values for ITOT and IPRED for all settings of output level and pre-
emphasis. The predriver current always flows from VCC to VEE.
It is treated separately from the output current, which flows
from VTTO, and may not be the same voltage as VCC.
The final section is the outputs. For an individual output, the
programmed output current flows through two separate paths.
One is the on-chip termination resistor, and the other is the
transmission line and the destination termination resistor. The
nominal parallel impedance of these two paths is 25 Ω. The
sum of these two currents flows through the switches and the
current source of the AD8158 output circuit and out through VEE.
The power dissipated in the transmission line and the destination
resistor is not dissipated in the AD8158 but has to be supplied
from the power supply and is a factor in the overall system
power. The current in the on-chip termination resistors and
the output current source dissipate power in the AD8158 itself.
OUTPUTS
The output current is set by a combination of output level and
pre-emphasis setting (see Table 20). For the two logic switch
states, this current flows through an on-chip termination
resistor and a parallel path to the destination device and its
termination resistor. The power in this parallel path is not
dissipated by the AD8158. With pre-emphasis enabled, some
current always flows in both the P and N termination resistors.
This pre-emphasis current gives rise to an output common-
mode shift, which varies with ac-coupling or dc-coupling and
which is calculated for both cases in Table 20.
Perhaps the most direct method for calculating power dissi-
pated in the output is to calculate the power that would be
dissipated if all of ITOT were to flow on-die from VTTO to VEE
and to subtract from this the power dissipated off-die in the
destination device termination resistors and the channel.
For this purpose, the destination device and channel can be
modeled as 50 Ω load resistors, RL, in parallel with the AD8158
termination resistors.
POWER SAVING CONSIDERATIONS
While the AD8158 power consumption is very low compared to
similar devices, careful control of its operating conditions can
yield further power savings. Significant power reduction can be
realized by operating the part at a lower voltage. Compared to
3.3 V operation, a supply voltage of 1.8 V can result in power
savings of ~45%. There is no performance penalty when
operating at lower voltage.
A second measure is to disable transmitters when they are not
being used. This can be done on a static basis if the output is
not used or on a dynamic basis if the output does not have a
constant stream of traffic. On transmit disable (Register 0x48,
Register 0x88, Register 0xC8), both the predriver and output
switch currents are disabled. The LOS-activated squelch
disables only the output switch current, ITOT. Superior power
saving is achieved by using the TX and RX disable registers to
turn off an unused lane as opposed to relying on the AD8158
transmit squelch feature.
Because the majority of the power dissipated is in the output
stage, some of its flexibility can be used to lower the power
consumption. First, the output current and output pre-emphasis
settings can be programmed to the smallest amount required to
maintain BER performance. If an output circuit always has a
short length and the receiver has good sensitivity, then a lower
output current can be used.
It is also possible to lower the voltage on VTTO to lower the
power dissipation. The amount that VTTO can be lowered is
dependent on the lowest of all the output’s VOL and VCC. This
is determined by the output that is operating at the highest
programmed output current. Table 1 and Table 20 list minimum
output levels.
Rev. 0 | Page 28 of 36
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