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AD8145YCPZ-R2 Datasheet(PDF) 16 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) 16 Page - Analog Devices

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AD8145
Rev. A | Page 16 of 24
To achieve unity gain from VREF to VOUT in this configuration,
divide VREF by the same factor used in the feedback loop; the
divider resistors, RD, need not be the same values used in the
internal feedback loop. Figure 35 illustrates this approach.
GAIN
REF
R
R
0.01µF
0.01µF
+5V
–5V
C
VREF
VOUT
VIN
RD
RD
Figure 35. Basic Gain Circuit: VOUT = 2VIN + VREF
The gain equation for the circuit in Figure 35 is
VOUT = 2VIN + VREF
(3)
Another configuration that provides the same gain equation as
Equation 3 is shown in Figure 36. In this configuration, it is
important to keep the source resistance of VREF much smaller
than 350 Ω to avoid gain errors.
GAIN
REF
R
R
0.01µF
0.01µF
+5V
–5V
C
VREF
VOUT
VIN
Figure 36. Basic Gain Circuit: VOUT = 2VIN + VREF
For stability reasons, the inductance of the trace connected to
the REF_x pin must be kept to less than 10 nH. The typical
inductance of 50 Ω traces on the outer layers of the FR-4 boards
is 7 nH/in, and on the inner layers, it is typically 9 nH/in. Vias
must be accounted for as well. The inductance of a typical via in
a 0.062-inch board is 1.5 nH. If longer traces are required, a 200 Ω
resistor should be placed in series with the trace to reduce the
Q-factor of the inductance.
In many dual-supply applications, VREF can be directly connected to
ground right at the device.
TERMINATING THE INPUT
One of the key benefits of the active feedback architecture is the
separation that exists between the differential input signal and
the feedback network. Because of this separation, the differential
input maintains its high CMRR and provides high differential
and common-mode input impedances, making line termination
a simple task.
Most applications that use the AD8145 involve transmitting broad-
band video signals over 100 Ω UTP cables and use dc-coupled
terminations. The two most common types of dc-coupled
terminations are differential and common-mode. Differential
termination of 100 Ω UTP cables is implemented by simply
connecting a 100 Ω resistor across the amplifier input, as shown
in Figure 37.
GAIN
REF
R
R
0.01µF
0.01µF
+5V
–5V
C
OUT
VOUT
VIN
100
UTP
100
Figure 37. Differential-Mode Termination with G = 1
Some applications require common-mode terminations for
common-mode currents generated at the transmitter. In these
cases, the 100 Ω termination resistor is split into two 50 Ω
resistors. The required common-mode termination voltage is
applied at the tap between the two resistors. In many of these
applications, the common-mode tap is connected to ground
(VTERM (CM) = 0). This scheme is illustrated in Figure 38.
GAIN
REF
R
R
0.01µF
0.01µF
+5V
–5V
C
OUT
VOUT
VIN
100
UTP
50
50
VTERM(CM)
Figure 38. Common-Mode Termination with G = 1


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