Video buffer required

Search for "Field Memory" or "Frame Memory" or "Frame store"

Logic Devices were going to bring out a configurable frame store device.

If you find field memory remember it most likely is designed for interlaced video and may need higher speed for non-interlaced VGA video stream, or possibly doubled up. Most field memory chips are fairly cheap as they are designed for VCRs and TVs.

Failing that high speed RAM and a PLD/FPGA to control the addressing and dual porting.

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Robert

Thanks for the suggestion but I've had a look at Maxim parts. The supply voltage is a bit of a problem in that I only have a maximum of 3.3V available. I've found a few muxes by other manufacturers (Intersil, AD) e.g. a triple 2:1, but I need a 1:2 selector which don't seem to be around or separate video buffers that can drive one output each and either select one or have both running all the time. Again I haven't found a suitable one yet. Thanks again.

Rog.

Last time I used any RGB muxes for 2:1 was a maxim part at +/-5V the MAX463 which suited the application as the opamps were using the same rails and got me better full swing on video signals for digitisation. Normally used AD8052 op-amps for the video buffering.

I am not sure I understand the rest of your requirements about "both [outputs] running all the time".

The MAX463 is a triple 2:1 video mux, last I used I got direct from Maxim and Farnell.

Thanks but a mux isn't what I need. I need either a selector or have 2 amps running from the DAC and drive both outputs all the time.

Thanks,

Rog.

Please reply AFTER the trimmed quote as this is getting difficult to follow.

If you have two outputs I would normally put two sets of amps in to drive each as these types of monitors cannot be chained like normal video monitors. This is assuming you want the SAME image on two monitors.

If you need to turn an output off or blank it, consider either using a small crosspoint switch or extending clamping controls to blank signal.

Otherwise your choice is either use video amp chips (aka Maxim MAX496) or your own amps. Watch the MIN output current on video drivers as these assume a permanent DC path not always shown in datasheet example circuits. Especially when no cable is connected.

Considering a 75R back terminated video feed should be driving at a max of 2/150 = 13mA, anything requiring a minimum output current more than that needs extra items on the output. Been caught by that one before.

As this is taking a VGA type output to (assumedly) drive two VGA type monitors, get high bandwidth opamps, bearing in mind the dot clock speed of the resolution you are driving at. For straight VGA (640 x 480 non-interlaced) anything over 100MHz bandwidth will do for short cable runs.

Unless you have long cables and have to drive and compensate for cables opamps will do, needs some external resistors for gain and back termination to do the job quite easily.

If you only have 0V and 3.3V as your rails ensure your video is offset from

Opamps like AD8052 work on 3.3V perfectly OK, and are speced on single 3.3V rail as well.

I generally run video circuits where possible from +/-5V to allow for easy ground referencing, clamping and allow for any amplification factor likely to be required (one ADC required the input to be offset by about 1.4V and have a 2V swing, from 1.4 to 3.4V).

If handheld/battery device you should put AC coupling on outputs. The norm for video is mains driven equipment DC couple outputs and AC couple inputs, to avoid too much drift and coupling effects.

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