alec(at)alecmyers.com Guest
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Posted: Thu Oct 19, 2017 8:43 am Post subject: Constructing an automatic RCA video camera feed splitte |
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How about floating the PIC power rails to -1.8V/+1.8V ? Or +2.5/-2.5V You’re going to have to generate a PIC voltage supply somewhere, regardless.
On Oct 19, 2017, at 10:36 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com> wrote:
At 07:45 AM 10/19/2017, you wrote:
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I think weâre ok. That section is either wrong, or badly written, I canât work out what itâs trying to say either. But:
Look at table 1 in para 8.4 - control L gives High-Z at the output.
9.2.1 says âAvoid bus contention because it can drive currents in excessof maximum limits.â - itâs clearly anticipated as a bus driver IC.
On the left of figure 16 shows output pins 2,3,9 and 10 tied together to form the output of the 4-channel mux in the sample application circuit.
AD8184 also needs a split supply. I donât think youâll find any CMOS devices that will switch an analog signal outside the supply rail, itâs not compatible with the tech.
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Agreed . . . I think the 4066 will work . . . and
unlike my earlier assertion about paralleling
outputs of a PIC, this chip's outputs go to
a hi-z state when OFF meaning that an unselected
channel will not load the output . . . further
more than one channel can be on at a time when the
application calls for mixing the two signals.
The 4066 would be difficult to control in a bi-
polar supply . . . but a single 5v supply should
suffice.Your signal of interest has to stay between
the rails. Standard NTSC video is 1 volt pk-pk. The
cameras I looked at seemed to be happy running
from 3.3v supplies . . . hence quite capable of
1v pk-pk output.
We don't know if this cameral video is capacity
coupled (floating DC reference) or DC stable. I suspect
it is capacity coupled; not DC stabilized. So
you might want to put 1K/1K voltage dividers on
the analog inputs to the 4066 to keep the 1v pk-pk
signal centered between the rails. If the cameras
are DC stable, they probably DC restored on the
lowest excursion of the sync pulse so that the
range of video signal voltages is more positive
than zero and much less than 3.3v. So all you
need to worry about is keeping the signal pretty
much centered between the Vss/Vdd rails.
Bob . . .
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