
Preliminary Technical Data
AD8176
terminated CAT5 cable (25 Ω effective impedance to ground at
each input pin, or 100 Ω differential source impedance across
pairs of input pins). If a different source impedance is presented,
the differential gain of the AD8176 can be calculated as
kΩ
525
.
Rev. PrA | Page 25 of 32
S
DM
R
G
+
=
kΩ
5
where
R
S
is the effective impedance to ground at each input pin.
When operating with a differential input, care must be taken to
keep the common-mode, or average, of the input voltages
within the linear operating range of the AD8176 receiver. For
the AD8176 receiver, this common-mode range can extend rail-
to-rail, provided the differential signal swing is small enough to
avoid forward biasing the ESD diodes (it is safest to keep the
common-mode plus differential signal excursions within the
supply voltages of the part).
The input voltage of the AD8176 is linear for ±1 V of differential
input voltage difference (this limitation is primarily due to
ability of the output to swing close to the rails, since the
differential gain through the part is +2). Beyond this level, the
signal path will saturate and limit the signal swing. This is not a
desired operation, as the supply current will increase and the
signal path will be slow to recover from clipping. The absolute
maximum allowed differential input signal is limited by long-
term reliability of the input stage. The limits in the Absolute
Maximum Ratings section of the data sheet should be observed
in order to avoid degrading device performance permanently.
AC Coupling
It is possible to ac-couple the inputs of the AD8176 receiver, so
that bias current does not need to be supplied externally. A
capacitor in series with the inputs to the AD8176 creates a high-
pass filter with the input impedance of the device. This
capacitor needs to be sized large enough so that the corner
frequency includes all frequencies of interest.
Differential Output
Benefits of Differential Operation
The AD8176 has a fully-differential switch core with differential
outputs. The two output voltages move in opposite directions,
with a differential feedback loop maintaining a fixed output
stage differential gain of +2. This differential output stage
provides improved crosstalk cancellation due to parasitic
coupling from one output to another being equal and out of
phase. Additionally, if the output of the device is utilized in a
differential design, then noise, crosstalk, and offset voltages
generated on-chip that are coupled equally into both outputs
are cancelled by the common-mode rejection ratio of the next
device in the signal chain. By utilizing the AD8176 outputs in a
differential application, the best possible noise and offset
specifications can be realized.
Differential Gain
The specified signal path gain of the AD8176 refers to its
differential gain. For the AD8176, the gain of +2 means that the
difference in voltage between the two output terminals is equal
to twice the difference between the two input terminals.
Common-Mode Gain
The common-mode, or average voltage pairs of output signals is
set by the voltage on the VOCM_CMENCOFF pin when
common-mode encoding is off (CMENC is a logic low), or by
the voltage on the VOCM_CMENCON pin when common-
mode encoding is on (CMENC is a logic high). Note that in the
latter case, VCOM_CMENCON sets the overall common-mode
of RGB triplets of differential outputs, while the individual
common-mode of each RGB output is free to change.
VCOM_CMENCON and VCOM_CMENCOFF are typically
set to midsupply (often ground), but can be moved
approximately ±0.5 V in order to accommodate cases where the
desired output common-mode voltage may not be midsupply
(as in the case of unequal split supplies). Adjusting the output
common-mode voltage beyond ±0.5 V can limit differential
swing internally below the specifications on the data sheet. The
overall common-mode of the output voltages follow the voltage
applied to VOCM_CMENCON or VCOM_CMENCOFF,
implying a gain of +1. Likewise, sync-on common-mode
signaling is carried through the AD8176 (CMENC must be in
its high state), implying a gain of +1 for this path as well.
The common-mode reference pins are analog signal inputs,
common to all output stages on the device. They require only
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 AD8176 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 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.