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AD8145YCPZ-R7 Datasheet(PDF) 18 Page - Analog Devices

Part No. AD8145YCPZ-R7
Description  High Speed, Triple Differential Receiver with Comparators
Download  24 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com

AD8145YCPZ-R7 Datasheet(HTML) 18 Page - Analog Devices

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Rev. 0 | Page 18 of 24
Figure 41 shows a specific example of external common-mode
Figure 41. External Common-Mode Clamping
The series resistances, RS, limit the current in each leg,
and the Schottky diodes limit the voltages on each input to
approximately 0.3 V to 0.4 V over the positive power supply,
V+, and to 0.3 V to 0.4 V below the negative power supply, V−.
The maximum value of RS is determined by the required signal
bandwidth, the line impedance, and the effective differential
capacitance due to the AD8145 inputs and the diodes.
As with the differential clamp, the series resistors should be
highly matched in value to preserve high frequency CMRR.
The two most important issues with regard to printed circuit
board (PCB) layout are minimizing parasitic signal trace
reactances in the feedback network and providing sufficient
thermal relief.
Excessive parasitic reactances in the feedback network cause
excessive peaking in the frequency response of the amplifier
and excessive overshoot in its step response due to a reduction
in phase margin. Oscillation occurs when these parasitic
reactances are increased to a critical point where the phase
margin is reduced to zero. Minimizing these reactances is
important to obtain optimal performance from the AD8145.
General high speed layout practices should be adhered to when
applying the AD8145. Controlled impedance transmission lines
are required for incoming and outgoing signals, referenced to a
ground plane.
Typically, the input signals are received over 100 Ω differential
transmission lines. A 100 Ω differential transmission line is
readily realized on the printed circuit board using two well-
matched, closely-spaced, 50 Ω single-ended traces that are
coupled through the ground plane. The traces that carry the
single-ended output signals are most often 75 Ω for video
signals. Output signal connections should include series
termination resistors that are matched to the impedance of the
line they are driving. When driving high impedance loads over
very short traces, impedance matching is not required. In these
cases, small series resistors should be used to buffer the
capacitance presented by the load.
Broadband power supply decoupling networks should be placed
as close as possible to the supply pins. Small surface-mount
ceramic capacitors are recommended for these networks, and
tantalum capacitors are recommended for bulk supply
Minimizing Parasitic Feedback Reactances
Parasitic trace capacitance and inductance are both reduced in
the unity-gain configuration when the feedback trace that
connects the OUT pin to the GAIN pin is reduced in length.
Removing the copper from all planes below the trace reduces
trace capacitance, but increases trace inductance, since the loop
area formed by the trace and ground plane is increased. A
reasonable compromise that works well is to void all copper
directly under the feedback trace and component pads with
margins on each side approximately equal to one trace width.
Combining this technique with minimizing trace length is
effective in keeping parasitic trace reactance in the unity-gain
feedback loop to a minimum.
Maximizing Heat Removal
A 5 × 5 array of thermal vias works well to connect the exposed
paddle to internal ground planes. The vias should be placed
inside the PCB pad that is soldered to the exposed paddle, and
should connect to all ground planes.
The AD8145 includes ground connections on its corner pins.
These pins can be used to provide additional heat removal from
the AD8145 by connecting them between the PCB pad that is
soldered to the exposed paddle and a ground plane on the
component side of the board. This layout technique lowers the
overall package thermal resistance. Use of this technique is not
required, but it does result in a lower junction temperature.
Designs must often conform to design for manufacturing
(DFM) rules that stipulate how to lay out PCBs in such a way as
to facilitate the manufacturing process. Some of these rules
require thermal relief on pads that connect to planes, and the
rules may limit the extent to which this technique can be used.

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