AD9884A: Crosstalk from HSYNC and VSYNC to the HSOUT line

Document created by analog-archivist Employee on Feb 23, 2016
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I use the AD9884A for a RGB video signal with SOG.
pixelclock : 30Mhz.
VS : 60Hz
HS : 26,4 kHz
I use an external sync slicer to extract HS and VS from G (both are negative
polarity).
I use an external coast generator.
I want to use HSOUT from the ADC for the rest of the system.

the problem is that there are glitches on HSOUT. They appears around the VS and
at each falling edge of the HSYNC input.
These glitches also disturbe the G input :
When the G signal is connected to the ADC, this signal is disturbed.
When I remove the 47nF capacitor (G signal disconnected from ADC), the G signal
is not anymore disturbed.

 

The HSOUT on the AD9884A should produce a nice clean signal. You may wish to
refer to the eval board. From your description, it sounds like you are getting
some cross-talk from the HSYC and VSYNC signals on the to G signal and this in
turn is coupling into the HSOUT. It is possible but unlikely that this
cross-talk is happening in side the AD9884A chip.

I suggest you review your PCB layout. Ensure that digital signals are
adequately separated from the analogue video signals, ideally video signals
should be shielded on all sides by adjacent grounded conductors. Ensure you
have adequate grounding. Use of ground planes is mandatory with the AD9884A. If
using a 2 layer board top and bottom should be filled with ground plane.
Preferably use a 4 layer board with one dedicated ground plane, split for
analog and digital return currents. All GND pins on the AD9884A should be
connected to the analog ground plane of your system and routed separately back
to the power supply via a low impedance ground plane. All digital circuitry
should be grounded to the digital ground plane in your system, it is critical
that the high frequency ground return current does not flow under the sensitive
analog circuits of the ad9884a.

The final chapter in all our seminar books is dedicated to hardware design
techniques and deals with such issues as grounding, decoupling, parasitic
thermocouples and good PCB design.
http://www.analog.com/support/standard_linear/seminar_material/index.html

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