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Dual Analog video display from single THS8200 DAC

Bob Sachsalber
Prodigy 60 points
Other Parts Discussed in Thread: THS8200, THS7316, THS7320, THS7365, THS7373

Hello,

I have a requirement to convert a Digital 24-bit RGB +HVsync to RGB analog with separate digital HVsync signal.   The display resolution is fixed at either 854x480 or 854x600 progressive scan at 60Hz.  I believe this resolution lies between EDTV and HDTV.

The analog signal to the display must be 0V to 0.7V peak to peak with no offset for blanking or sync, as the sync will be driven separately by the THS8200.

I am considering using one of the THS731x family of devices with internal LPF but because of the display requirements not sure which device would be best suited.  This device needs to be very small and low powered and powered from a single 3v3 power rail.

The displays will be located fairly close to the op amps about 4 inches, my preference would be to have DC coupling between the displays and the op amps.

Thanks,

Bob

  • 0
    TI__Expert 3030 points

    Bob,

    I was looking on VESA document I have (DMT 1.0 Rev 11 - 2007) and only see 800x600 and 848x480. 848x480 @ 60Hz is 33.75MHz Pixel Rate while 800x600 @ 60Hz is 40MHz refresh rate. I suspect the 854x480 or 854x600 will be around these pixel rates. As such, I would recommend the THS7316 as it passes up to 30MHz (~60MHz pixel rate area) and should meet the other functional requests.

    Cheers,

    Randy

  • 0 in reply to Randy S
    Prodigy 60 points

    Hello Randy,

    Thanks for your recommendation, I should have mentioned that the Display manufacturer has provided the video signal specifications for the 854x600p60 display characteristics.  The Pclk is 41.707MHz with HSync at 37.642KHz and VSync at 59.94Hz.

    My application is a portable head worn device and both size and power matter.  I have also been looking at the THS7319 and THS7320 for these specific reasons.  Are these not suitable because of their fixed reconstruction filters?  Or is the roll off not steep enough?

    Cheers,

    Bob

  • 0 in reply to Bob Sachsalber
    TI__Expert 3030 points

    The THS7319 may work, but the picture quality may not be ideal due to the filter characateristics of the fixed filter. It is -3dB down at about 20MHz which may not give the absolute sharpest image when a step occurs (ex: 0IRE to 100IRE level step response). Don't get me wrong, it will look pretty good, but not "perfect". For the size and power the THS7319 gives, it may be worthwhile to look at the THS7319.

    For the "best" picture quality, the THS7316 (or the 3-HD channels of the THS7373 or THS7365) would be the best choices.

    Cheers,

    Randy

  • 0 in reply to Randy S
    Prodigy 60 points

    Thanks Randy I will look at these devices more closely.

    One final question/concern.  These devices have a feature called DC-shift, in my case the output from the op-amp should supply 0V - 0.7V video signal and the sync is provided separately on two logic side band signals HSync and VSync.   Or can I drive the display with AC coupling and still have the same performance.  I would prefer to use DC coupling because the video signal remains inside our display package and the interconnect is via a short flex pcb cable of 4 inches.

    Thanks again,

    Bob

  • 0 in reply to Bob Sachsalber
    TI__Expert 3030 points

    Bob,

    The reason these parts have a DC level shift is to mitigate any potential saturation/clipping effects when the signal is the lowest level. These are single supply "op-amps" which ahave rail-to-rail outputs, but this means it gets close to the rail (GND) but never at the rail.

    I would recommend DC coupling. It is easiest and should work without issue.

    Just about everything out there today uses a ADC/Decoder/SOC which normally has an input coupling capacitor on it's front-end and the chip will have it's own DC restoration/sync-tip clamp/back-porch clamp functionality. This will re-establish the dc operating point of the signal to what the chip needs. As such, a small DC level shift on the output of the amplifier should have no effect on the picture. This is also why if you AC couple the output of the amplifier, it will still look the same on the display as the front-end chip of the display AC couples anyways.

    Cheers,

    Randy

  • 0 in reply to Randy S
    Prodigy 60 points

    Randy,

    I will investigate the use of AC coupling between the DAC and op-amp, if I understand your last paragraph correctly.  The design team is leaning toward the THS7316 as the first choice.

    Would it be possible to provide you with the schematic design for review?  Or do I need to arrange that type of assistance through Distribution FAE?

    Cheers,

    Bob

  • 0 in reply to Bob Sachsalber
    TI__Expert 3030 points

    Bob,

    The inputs to all THS73xx parts will support input signals even below GND. I was more focused on the output side discussion and output AC coupling more than input coupling. The only reason I can see to use AC coupling on the input side between the DAC and the THS73xx inputs is if it is a current-sink DAC where the termination resistor is connected to the positive supply, or if the voltage levels out of the DAC are higher than say 1.5Vpk. Note that the THS7319 does not have a sync-tip clamp function on the front-end and AC input coupling of this part would be limited to AC-Biasing only.

    I can certainly review the schematic. It is your choice, but if you want to send it to me privately send it to r-stephens@ti.com for review.

    Cheers,

    Randy