Fail-safe VGA buffer

[F. Bertolazzi]

I need to split a SXGA (1280x1024, 500 MHz) signal so that I can drive two
monitors. Trivial, you'll say. Buy a ready-made one or take an AD8074 or
similar and follow the reference design.

The catch is that the first monitor must be visible whatever happens
(electrically) to the splitter. The second monitor is "expendable", well,
not the monitor itself, it's image.

How can I isolate the input of the video amplifier so that it cannot
interfere appreciably with the signal?

 

[F. Bertolazzi]

Jan Panteltje:

relay

Which sort?

 

[F. Bertolazzi]

Jan Panteltje:

low capacity.

In the meanwhile I found some "coaxial" relays from Farnell that are made
for that purpose. Thanks.

Any better idea? 10 such relays cost quite a bit, occupy a lot of real
estate, consume, need to be driven by something that detects signal loss,
and their weight do not allow me to make a small adapter that could be
placed in the back of the monitor.

 

[F. Bertolazzi]

Jan Panteltje:

If I was in China, then I would make some small spring metal strips that
would contact the PCB... with a coil to lift those if power...

Well, I might think about it, since it would be operated sparingly if at
all, but I need a DPDT for each of the 5 channels for disconnecting both
input and outputs.

There are non-coaxial relays with very low capacitance too.

But dimensions and prices are about the same.

Do you think I'm going to waste my time if I try to experiment with a video
op-amp in inverting configuration with an input resistor of about 1k?

The 1k resistor would be a good enough "isolation".

 

[Nico Coesel]

Jan Panteltje:


Which sort?

Almost any small signal relay will do. Just make sure the users choose
a 60Hz refreshrate. Higher is no use with today's flat panel monitors.

 

[F. Bertolazzi]

Jan Panteltje:

Without power the chips must be able to handle current (in the limiting diodes).

One particular op amp claims that its impedance goes very high without
power, but the feedback resistor remains.

OTOH 1 k at 500 MHz needs only a small cap to form a lowpass.
I dunno the input capacitance of your chips.
Check that RC time.

The datasheet says that the non-inverting input has 1 pF capacitance, that
would amount to about 160 MHz. :-(

And anyway the 0.1 dB gain flatness, that I read somewhere is the maximum
allowable for good results, is up to just 100 MHz, at 400 MHz is -3 dB.

Geez, I hate relays.

 

[F. Bertolazzi]

[email protected]:

why both in and output?

The thing must be fail-safe.

how about fig.3 in the ad8075 and relays on the supply pins ?

Because a broken or unpowered AD8075 would kill the signal.

Or not?

 

[Cydrome Leader]

If I was in China, then I would make some small spring metal strips that
would contact the PCB... with a coil to lift those if power...
There are non-coaxial relays with very low capacitance too.

Don't forget the plastic springs.

 

[F. Bertolazzi]

Cydrome Leader:

Don't forget the plastic springs.

In the coax relays?

 

[F. Bertolazzi]

Jan Panteltje:

Battery backup?
Li-ion?

I'm afraid that's not enough. I will check with the customer, though.
But there is the usual governmental Agency hanging around, that, since they
have no idea of what they really want (apart from annoying people), will
certainly want something impossible, or expensive, or both.

 

[F. Bertolazzi]

whit3rd:

Can you run your source to splitter A, screen #1 on output A1,
splitter B on output A2? Then the splitter B can collect bullet
holes, and
you don't lose image on screen #1.

The problem are the bullets into splitter A. Anything we connect to the VGA
cable must be harmless to the signal.

On the other hand, you may say, if a relay does not work...

 

[Grant]

Battery backup?
Li-ion?

Long time ago I made a box that uses a pair of 4-way signal relays to switch
VGA video signal from one of two sources to monitor, I don't see any video
artifacts (no obvious smearing of vertical lines) from the signals going
through the relays. The relays are 4-way quality telecomms style.

Grant.

 

[F. Bertolazzi]

Grant:

Long time ago I made a box that uses a pair of 4-way signal relays to switch
VGA video signal from one of two sources to monitor, I don't see any video
artifacts (no obvious smearing of vertical lines) from the signals going
through the relays. The relays are 4-way quality telecomms style.

Can you please tell me the model or at least the brand?

 

[F. Bertolazzi]

Nico Coesel:

Almost any small signal relay will do.

The ones I found are priced similarly and have the same dimensions.

Just make sure the users choose a 60Hz refreshrate.

I can't.

Thanks.

 

[Nico Coesel]

Nico Coesel:


The ones I found are priced similarly and have the same dimensions.


I can't.

Why not? What kind of refreshrate are they using?

 

[F. Bertolazzi]

Nico Coesel:

Why not? What kind of refreshrate are they using?

I don't know. I may ask, but I found some 2 GHz relays, $5 apiece that will
do the job at any refresh rate.

 

[Grant]

Grant:


Can you please tell me the model or at least the brand?

Sorry for delay:

| [m]Aromat 9604A
| DS4E-S-DC12V 0.6A 125V AC
| 0.6A 110V DC
| 2A 30V DC
| MADE IN USA (UR) (SA)

The [m] is a logo looks like upside down 'w' on black rectangular background,
(UR) is that ubiquitous safety mark, the (SA) is 'SA' inside a 'C' bottom right
corner. Plastic case is yellow top, black underneath.

I can post photos if you need more info.

Grant.

 

[F. Bertolazzi]

Ken S. Tucker:

A friend of mine (son is a systems analyst as a hobby)
has 4 monitors off one computer.
The CPU (Quadcore 2 slot PCI express) has two ATI video
cards, each of which drivers 2 monitors, he does all the switching
by software.
If that's what you want I can connect you two.
Ken

Thanks, but what I need to connect is a naval radar to a second S-VDR (a
"black box" recorder).

 

[F. Bertolazzi]

Grant:

| [m]Aromat 9604A
| DS4E-S-DC12V 0.6A 125V AC

Thanks. It's similar to the ones I found.