參數(shù)資料
型號(hào)： AD8178ABPZ
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 25/40頁(yè)
文件大?。?/td> 0K
描述： IC VIDEO CROSSPOINT SWIT 676BGA
標(biāo)準(zhǔn)包裝： 1
功能： 視頻交叉點(diǎn)開關(guān)
電路： 3 x 16:5
電壓電源： 單/雙電源
電壓 - 電源，單路/雙路(±)： 4.5 V ~ 5.5 V，±2.5V
電流 - 電源： 460mA
工作溫度： -40°C ~ 85°C
安裝類型： 表面貼裝
封裝/外殼： 676-BGA
供應(yīng)商設(shè)備封裝： 676-BGA（27x27）
包裝： 管件
AD8178
Rev. 0 | Page 31 of 40
small amounts of bias current, but noise appearing on these
pins is buffered to all the output stages. As such, they should
be connected to low noise, low impedance voltage references
to avoid being sources of noise, offset, and crosstalk in the
signal path.
Termination
The AD8178 is designed to drive 100 Ω terminated to ground
on each output (or an effective 200 Ω differential) while
meeting data sheet specifications over the specified operating
temperature range, if care is taken to observe the maximum
power derating curves.
Termination at the load end is recommended to shorten settling
time and provide for best signal integrity. In differential signal
paths, it is often desirable to series-terminate the outputs, with a
resistor in series with each output. A side effect of termination
is an attenuation of the output signal by a factor of two. In this
case, gain is usually necessary somewhere else in the signal path
to restore the signal level.
Whenever a differential output is used single-ended, it is desirable
to terminate the used single-ended output with a series resistor,
as well as to place a resistor on the unused output to match the
load seen by the used output.
When disabled, the outputs float to midsupply. A small current
is required to drive the outputs away from their midsupply state.
This current is easily provided by an AD8178 output (in its
enabled state) bussed together with the disabled output.
Exceeding the allowed output voltage range may saturate
internal nodes in the disabled output, and consequently,
an increase in disabled output current may be observed.
Single-Ended Output
Usage
The AD8178 output pairs can be used single-ended, taking only
one output and not using the second. This is often desired to
reduce the routing complexity in the design or because a single-
ended load is being driven directly. This mode of operation
produces good results but has some shortcomings when compared
to taking the output differentially. When observing the single-
ended output, noise that is common to both outputs appears in
the output signal.
When observing the output single-ended, the distribution of offset
voltages appears greater. In the differential case, the difference
between the outputs, when the difference between the inputs is
zero, is a small differential offset. This offset is created from
mismatches in devices in the signal path. In the single-ended
case, this differential offset is still observed, but an additional
offset component is also relevant. This additional component
is the common-mode offset, which is the difference between the
average of the outputs and the output common-mode reference.
This offset is created by mismatches that affect the signal path
in a common-mode manner. A differential receiver rejects this
common-mode offset voltage, but in the single-ended case, this
offset is observed with respect to the signal ground. The single-
ended output sums half the differential offset voltage and all
of the common-mode offset voltage for a net increase in
observed offset.
Single-Ended Gain
The AD8178 operates as a closed-loop differential amplifier.
The primary control loop forces the difference between the
output terminals to be a ratio of the difference between the
input terminals. One output increases in voltage, while the
other decreases an equal amount to make the total output
voltage difference correct. The average of these output voltages
is forced to the voltage on the common-mode reference terminal
(VOCM_CMENCOFF or VOCM_CMENCON) by a second
control loop. If only one output terminal is observed with respect
to the common-mode reference terminal, only half of the differ-
ence voltage is observed. This implies that when using only one
output of the device, half of the differential gain is observed.
An AD8178 taken with single-ended output appears to have
a gain of +2.
It is important to note that all considerations that apply to the
used output phase regarding output voltage headroom apply
unchanged to the complement output phase, even if this is not
actually used.
Termination
When operating the AD8178 with a single-ended output, the
preferred output termination scheme is to refer the load to the
output common mode. A series termination can be used, at an
additional cost of one half the signal gain.
In single-ended output operation, the complementary phase of
the output is not used and may or may not be terminated locally.
Although the unused output can be floated to reduce power dissi-
pation, there are several reasons for terminating the unused output
with a load resistance matched to the load on the signal output.
One component of crosstalk is magnetic coupling by mutual
inductance between output package traces and bond wires that
carry load current. In a differential design, there is coupling from
one pair of outputs to other adjacent pairs of outputs. The differ-
ential nature of the output signal simultaneously drives the
coupling field in one direction for one phase of the output and
in an opposite direction for the other phase of the output. These
magnetic fields do not couple equally into adjacent output pairs,
due to different proximities; but they do destructively cancel the
crosstalk to some extent. If the load current in each output is equal,
this cancellation is greater and less adjacent crosstalk is observed
(regardless of whether the second output is actually being used).
A second benefit of balancing the output loads in a differential
pair is to reduce fluctuations in current requirements from the
power supply. In single-ended loads, the load currents alternate
from the positive supply to the negative supply. This creates a
parasitic signal voltage in the supply pins due to the finite
resistance and inductance of the supplies. This supply fluctuation
appears as crosstalk in all outputs, attenuated by the power
supply rejection ratio (PSRR) of the device.
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