Multiplexing THS8200 VGA output

FYI, I find THS7327 (with amps) or THS7347 (no amps) that will include muxing for HV (total RGBHV switching).  But my questions above remain.

Helmut,

The THS7347 appears to be more for dirivng ADC inputs.

1.  Recommed using 3.3V5V level tranlators on the THS8200 output syncs to the monitor.  See the TPD7S019 for an integrated solution for VGA port ESD protection, sync level translation, and I2C level tranlation (if needed for DDC).

2., 3. The TDP7S019 above will proivide addtional VGA port protection.

4. The digital mux for HS/VSync could be placed before the TPS7S019.   Try to find one that does does not have excessive prop delay to maintain decent alignment with the analog RGB.

The AUX in will probably have 5V syncs, so no level tranlation required.  Driving 2 monitors with one VGA source can lead to noise issues, so I prefer to see separate buffers for AUX out and THS8200 out.  Noise could be injected on the RGB signals from one monitor to the other from ADC sampling and DC-restore clamp circuitry.

ESD protection included in most devices is not adequate for more strenuous 8KV testing that is often required.  Your requirements will depend on your ESD testing requirements and how your product will be used in the field.

...such an eloquent and all-inclusive answer.

Thanks very much,

Helmut

Larry,

"Rejected" answer not because it's incorrect but because later ramifications fall out.

Excluding the THS8200 and OLED, I have to fit all this in a space about 1" x 2".

Losing the HV multiplexing of the THS7327 is costing me a lot of complication and board space.  Even though the THS7327 is intended to feed an ADC rather than buffer a DAC, do you think I might be able to use it to buffer a DAC?

Comparing the THS7303 used by the DM6467T EVM and the THS7327 that includes H & V, I see they have the same ac-bias input config ability, I2C mux control, and 9/16/35 MHz LPF on the buffer path.  So if the THS8200 drives the THS7327 and I use the THS7327 "buffer" output rather than "monitor" output, I'll get all the features in the THS7303 except for the output impedance network.  I ought to be able to recreate that myself.  I'll be driving the internal OLED full time with this.

I've got most of the other stuff figured out, except for tiny chip for H/V level shift coming in from VGA-IN, and perhaps a V-sync buffer going to VGA-OUT so that a short on that line won't kill internal V-sync to OLED.  (Similar situation to isolating VGA-OUT RGB from internal RGB to OLED.)

-Helmut

Larry, et al.:

What do you think of this.

Use two NXP IP4774CZ14's (0.25" x 0.31", 0.08sqin) for RGBHV ESD protection, one on VGA-IN and another on VGA-OUT.  These tie only passing lines with no inserted delay.  H sync, however, can use buffer with 6.5ns delay.  Just in case, I put optional 0-ohm resistor bypass so I can remove buffer and it's delay, while still keeping ESD protection.

Use two THS7303's (0.26" SQ, 0.07sqin) loading same THS8200 RGB outputs, thus isolating THS7303's outputs from THS8200 and each other.  Note THS7303 includes filter to follow THS8200's DACs.  One THS7303 selects RGB from either VGA-IN or THS8200.  Other THS7303 selects RGB only ever from THS8200, with other input unused.  Both controlled by I2C. (Note TPD7S019 provided ESD protection and level shifting, but no isolation.)

Now for H V sync isolation and level shifting, following paths parallel to the RGB paths through THS7303's.  Use a single SN74LVC257A (0.26" x 0.19", .05sqin).  4.6ns delay << 40ns VGA pixel time.  That's a quad 2:1 mux with single control.  Feed control from PSoC elsewhere on board, that has I2C input and can produce select output.  Use two muxes to feed OLED, selecting H and V between THS8200 and protected VGA-IN.  Use the other two muxes in the opposite path direction, feeding the protection for VGA-OUT, but connecting BOTH inputs to THS8200 output H and V, since I don't want to switch at all but always feed THS8200 output to VGA-OUT.  I could have used a separate chip, but it takes more space.  This chip has only a single select.  So when select switches for purpose of feeding OLED, the use of duplicate inputs makes sure VGA-OUT always gets same THS8200 signals.

Total outside of THS8200 and OLED uses only 5 chips.  Total 0.35sqin.  Along with supporting R's C's and routing, should fit single sided on 1" x 2".  Have PSoC on there, too, and some other stuff.  So all should still fit.

-Helmut

ack.  thx.

Larry (or others):

I'm finally back on this, actually designing the circuit diagram and board.  I discovered an error in my logic level translation for HS and VS.  So I've recast the circuit a bit, and gone back to your original TPD7S019 suggestion.  

Now the only gap I have in the design is, I must internally select between one pair of HS/VS (at 3.3V level from THS8200) and another pair of HS/VS (at 5V from AUX-IN via TPD7S019).  I must output HS/VS (at 5V going to my OLED).

Therefore, I'm looking for 2:1 selecting and level translation, or at least level compatibility at the same time, on two lines.  I just can't find a single chip solution.  The smallest area I find is two SN74LVC1T45 on each 3.3V H or S line, followed by a SN54HC157 selector.  Any better recommendation, especially if single chip?

Otherwise, I am wondering about the following.  My THS8200 3.3V level HS/VS lines also go out another TPD7S019 to my AUX-OUT.  In doing so, the TPD7S019 is translating them to 5V.  That's "outside world", however, and subject to ESD.  I'm wondering about turning those 5V signals back around and going directly into the SN54HC157 selector.  This handles the 3.3V-to-5V translation, and gives me a single-chip solution again (single-additional-chip, at least).

My concern is the ESD from outside world, and possibly feedback messing up HS/VS themselves from outside world.  Perhaps the TPD7S019 ESD protection on its own input protects other things on that input, like the video lines do, but I'm not sure.  Now my brain hurts and I have a meeting I have to leave for.  Your advice greatly appreciated.

Thanks,

Helmut