AD8187 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8187 Datasheet PDF : 20 Pages
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AD8186/AD8187
A dc restore circuit using the AD8187 is shown in Figure 11.
Two separate sources of RGB video are ac-coupled to the
0.1 µF input capacitors of the AD8187. The input points of
the AD8187 are switched to a 1.5 V reference by the ADG786,
which works in the following manner:
The SEL A/B signal selects the A or B inputs to the AD8187. It
also selects the switch positions in the ADG786 such that the
same selected inputs will be connected to VREF when EN is low.
During the horizontal interval, all of the RGB input signals are at
a flat black level. A logic signal that is low during HSYNC is
applied to the EN of the ADG786. This closes the switches
and clamps the black level to 1.5 V. At all other times, the switches
are off and the node at the inputs to the AD8187 floats.
There are two considerations for sizing the input coupling capaci-
tors. One is the time constant during the H-pulse clamping. The
other is the droop associated with the capacitor discharge due to the
input bias current of the AD8187. For the former, it is better to
have a small capacitor; but for the latter, a larger capacitor is better.
The ON resistance of the ADG786 and the coupling capacitor
forms the time constant of the input clamp. The ADG786 ON
resistance is 5 Ω max. With a 0.1 µF capacitor, a time constant
of 0.5 µs is created. Thus, a sync pulse of greater than 2.5 µs will
cause less than 1% error. This is not critical because the black
level from successive lines is very close and the voltage changes
little from line to line.
A rough approximation for the horizontal line time for a graphics
system is 30 µs. This will vary depending on the resolution and
the vertical rate. The coupling capacitor needs to hold the voltage
relatively constant during this time while the input bias current
of the AD8187 is discharging it.
The change in voltage is IBIAS times the line time divided by
the capacitance. With an IBIAS of 2.5 µA, a line time of 30 µs,
and a 0.1 µF coupling capacitor, the amount of droop is
0.75 mV. This is roughly 0.1% of the full video amplitude and
will not be observable in the video display.
High Speed Design Considerations
The AD8186/AD8187 are extremely high speed switching ampli-
fiers for routing the highest resolution graphic signals. Extra care
is required in the circuit design and layout to ensure that the full
resolution of the video is realized.
First, the board should have at least one layer of a solid ground
plane. Long signal paths should be referenced to a ground plane
as controlled-impedance traces. All bypass capacitors should be
very close to the pins of the part with absolutely minimum extra
circuit length in the path. It is also helpful to have a large VCC
plane on a circuit board layer that is closely spaced to the ground
plane. This creates a low inductance interplane capacitance,
which is very helpful in supplying the fast transient currents that
the part demands during high resolution signal transitions.
Evaluation Board
An evaluation board has been designed and is offered for run-
ning the AD8186/AD8187 on a single supply. The inputs and
outputs are ac-coupled and terminated with 75 Ω resistors.
For the AD8187, a potentiometer is provided to allow setting
VREF at any value between VCC and ground.
The logic control signals can be statically set by adding or
removing a jumper. If it is required to drive the logic pins
with a fast signal, an SMA connector can be used to deliver the
signal, and a place for a termination resistor is provided.
5V
VDD
ADG786
S1A
D1
S1B
5V
VREF
3.48k⍀ 1.5V
D2
+
1.5k⍀
S2A
S2B
2.4V MIN
10F 0.1F
S3A
D3
S3B
HSYNC
GND
VSS
LOGIC
EN A0 A1 A2
3V TO 5V 5V
0.1F
REDA
0.1F
GRNA
BLUA
0.1F
IN0A
IN1A
IN2A
DVCC VCC
AD8187
؋2
OUT0 RED
VREF
VREF
؋2
OUT1 GRN
0.1F
IN0B
REDB
0.1F
IN1B
GRNB
BLUB
IN2B
0.1F DGND VEE
؋2
OUT2 BLU
SEL A/B OE
0.8V MIN
SEL A/B
Figure 11. AD8187 AC-Coupled with DC Restore
–16–
REV. A
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