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Can someone take a look at this datasheet and tell me if I'm missing something?

A

ajcrm125

Jan 1, 1970
0
Okay... I'm trying to create a circuit that interrputs a video signal
to a monitor. (Right now I'm using an old arcade monitor and arcade
game to test). The part I chose was the TI OPA3692 tripple video amp
with disable. Here's the datasheet:
http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=opa3692

Basically all I did was tie -Vs to GND, +Vs to +5V, attach the RGB
signals from the game to +IN A/B/C respectively, and the OUT A/B/C
signals to the monitor. I left the -INA/B/C signals floating so that
the amps would provide a +1 gain as specified in the datasheet. When
the disable pin is high (deasserted) the video looks normal, but when I
ground it the video dims but is still very noticeable. According to
the datasheet, disabling the amps puts them in a high-impeadance state.
I even threw some 100k pulldowns in the outputs to the monitor but no
difference. Is there something I'm missing here?
Input voltage levels off? Can't think of a reason why the disable
function would not work....
Thanks for any help guys.
-Adam Courchesne
 
J

John Popelish

Jan 1, 1970
0
ajcrm125 said:
Okay... I'm trying to create a circuit that interrputs a video signal
to a monitor. (Right now I'm using an old arcade monitor and arcade
game to test). The part I chose was the TI OPA3692 tripple video amp
with disable. Here's the datasheet:
http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=opa3692

Basically all I did was tie -Vs to GND, +Vs to +5V, attach the RGB
signals from the game to +IN A/B/C respectively, and the OUT A/B/C
signals to the monitor. I left the -INA/B/C signals floating so that
the amps would provide a +1 gain as specified in the datasheet. When
the disable pin is high (deasserted) the video looks normal, but when I
ground it the video dims but is still very noticeable.

That Disable signal has a bar over it, meaning that the disable is
active when low.
According to
the datasheet, disabling the amps puts them in a high-impeadance state.
I even threw some 100k pulldowns in the outputs to the monitor but no
difference. Is there something I'm missing here?
Input voltage levels off? Can't think of a reason why the disable
function would not work....
Thanks for any help guys.

If your monitor has pretty high input impedance, then the signal is
jumping across the + to - input and going out the 402 ohm feedback
resistor. Try putting something like a 1k load across the output.

Normal video signals are 75 ohm impedance, so the 75 ohm input load
resistors, gain of 2 and 75 ohm series output resistors are typical,
with the monitor having another 75 ohms to ground at the monitor end
of the cable. But I suspect your monitor is a low bandwidth type
without the terminating resistors.
 
A

ajcrm125

Jan 1, 1970
0
That Disable signal has a bar over it, meaning that the disable is
active when low.
Right,... that's why I'm grounding it to try to disable the amps and
put them in a high impeadance state.
If your monitor has pretty high input impedance, then the signal is
jumping across the + to - input and going out the 402 ohm feedback
resistor. Try putting something like a 1k load across the output.
Jumping accross the + to - input? Never even crossed my mind. Is that
possible? I'm totally a digital guy so my analog experience is very
slim.
I thought that the inputs to an op amp were supposed to have a
virtually infinite resistance between them. (One of the only things I
recall from my analog course) :)

Nevertheless I'll give it a shot.... thanks!
 
J

John Popelish

Jan 1, 1970
0
ajcrm125 said:
Jumping accross the + to - input? Never even crossed my mind. Is that
possible? I'm totally a digital guy so my analog experience is very
slim.
I thought that the inputs to an op amp were supposed to have a
virtually infinite resistance between them. (One of the only things I
recall from my analog course) :)

There is a non zero capacitance across the two input pins. With a
reasonable output load, this current path is small.
 
A

ajcrm125

Jan 1, 1970
0
Ahhhhh.. gotcha. Good point.
I just had a revalation BTW... I think what I'll do is ground the -
inputs so the amp's gain is 2. Then I'll put a series resistor (as
suggested) accross the output pins, bringing the output back down to
proper elevels. That way, it will act as an amp with gain of 1 when
active and when in high impeadance, the internal resistors will be in
series to GND and should pull down the output for me.
Does that sound like it would work?
 
J

John Popelish

Jan 1, 1970
0
ajcrm125 said:
Ahhhhh.. gotcha. Good point.
I just had a revalation BTW... I think what I'll do is ground the -
inputs so the amp's gain is 2. Then I'll put a series resistor (as
suggested) accross the output pins, bringing the output back down to
proper elevels. That way, it will act as an amp with gain of 1 when
active and when in high impeadance, the internal resistors will be in
series to GND and should pull down the output for me.
Does that sound like it would work?

That is its optimum configuration. Don't forget the terminating
resistor between the + input and ground, to absorb the incoming
signal, so it doesn't bounce back and forth between the source and the
amplifier. That will cause thin ghosts at any high contrast vertical
edge. The monitor should also terminate its signals the same way.
 
A

ajcrm125

Jan 1, 1970
0
Good point. Thanks...
I looked at the monitor datasheet and schematic and it states (and
shows in the schematic) a 5.6k terminating resistor at the monitor.
Seems high but what do I know....
 
J

John Popelish

Jan 1, 1970
0
ajcrm125 said:
Good point. Thanks...
I looked at the monitor datasheet and schematic and it states (and
shows in the schematic) a 5.6k terminating resistor at the monitor.
Seems high but what do I know....
A modern, high resolution monitor could not get away with that kind of
terminator. This reminds me of the old black and white monitor (just
a stripped down black and white television) on my Radio Shack TRS-80.
 
A

ajcrm125

Jan 1, 1970
0
Yeah these monitors are really ooooooold school. :)
 
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