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AD8145YCPZ-R2 Datasheet(PDF) 19 Page - Analog Devices

Part No. AD8145YCPZ-R2
Description  High Speed, Triple Differential Receiver with Comparators
Download  24 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD8145YCPZ-R2 Datasheet(HTML) 19 Page - Analog Devices

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AD8145
Rev. A | Page 19 of 24
DRIVING A CAPACITIVE LOAD
The AD8145 typically drives either high impedance loads over
short PCB traces, such as crosspoint switch inputs, or doubly
terminated coaxial cables. A gain of 1 is commonly used in
the high impedance case because a 6 dB transmission line
termination loss is not incurred. A gain of 2 is required when
driving cables to compensate for the 6 dB termination loss.
In all cases, the output must drive the parasitic capacitance of
the feedback loop, conservatively estimated to be 1 pF, in addition
to the capacitance presented by the actual load. When driving a
high impedance input, it is recommended that a small series
resistor be used to buffer the input capacitance of the device
being driven. Clearly, the resistor value must be small enough to
preserve the required bandwidth. In the ideal doubly terminated
cable case, the AD8145 output sees a purely resistive load. In
reality, there is some residual capacitance, which is buffered by
the series termination resistor. Figure 42 illustrates the high
impedance case, and Figure 43 illustrates the cable driving case.
RS
CIN
GAIN
REF
R
R
0.01µF
0.01µF
+5V
–5V
C
OUT
VIN
VREF
Figure 42. Buffering the Input Capacitance of a High-Z Load with G = 1
GAIN
REF
R
R
0.01µF
0.01µF
+5V
–5V
C
VREF
VIN
RS
CS
RL
OUT
Figure 43. Driving a Doubly Terminated Cable with G = 2
POWER-DOWN
The power-down feature can be used to reduce power consumption
when a particular device is not in use and does not place the output
in a high-Z state when asserted. The power-down feature is
asserted when the voltage applied to the power-down pin drops
to approximately 2 V below the positive supply. The AD8145 is
enabled by pulling the power-down pin to the positive supply.
AUTOMOTIVE PRODUCTS
The AD8145W product is qualified per the AEC-Q100 for use
in automotive applications. This model meets stringent automotive
performance and quality requirements and offers the same
functionalities as the commercial and industrial grade devices but
operates over a temperature range of −40°C to +105°C. For more
information, contact your local Analog Devices, Inc., sales
representative.
COMPARATORS
In addition to general-purpose applications, the two on-chip
comparators can be used to decode video sync pulses from the
received common-mode voltages or to receive differential digital
information. Built-in hysteresis helps to eliminate false triggers
from noise.
The comparator outputs are designed to drive source-terminated
transmission lines. The source termination technique uses a resistor
in-series with each comparator output such that the sum of the
comparator source resistance (≈20 Ω) and the series resistor
equals the transmission line characteristic impedance. The load
end of the transmission line is high impedance. When the signal
is launched into the source termination, its initial value is one-
half of its source value because its amplitude is divided-by-2 by
the voltage divider formed by the source termination and the
transmission line. At the load, the signal experiences nearly
100% positive reflection due to the high impedance load and
is restored to nearly its full value. This technique is commonly
used in PCB layouts that involve high speed digital logic.
An internal linear voltage regulator derives power for the
comparators from the positive supply; therefore, the AD8145
must always have a minimum positive supply voltage of 4.5 V.


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