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Finding the video signal gremlin

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Microchip_Manny
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re: Finding the video signal gremlin
Microchip_Manny   7/27/2010 8:07:30 PM
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Hi Mprnprp, The images with the distortion was with the MegaOhm scope probe. Without geeking out too much, the issue lies in that the high signal levels involved the switching of larger current sources to drive out the desired level. Where at lower levels the amount of current sources being switched in, or out, were handled more smoothly with thermometer decoding. So just as there was an impedance mismatch with the MegaOhm scope probe there was a mismatch with the board trace to the target CMOS I.C. The DAC supported up to 250Msps rates, which requires fast settling times, so the edges where rather sharp; hence the antenna effects occurred that we were seeing.

rnprnprnp
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re: Finding the video signal gremlin
rnprnprnp   7/27/2010 10:28:19 AM
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What did the signal look like with a 1M ohm scope lead? Was it the circuit 50 ohm resistor that was the antenna or the 50 ohm scope lead in parallel?

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