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THS8135 Negative Sync

Noah Root
Prodigy 120 points
Other Parts Discussed in Thread: THS8135

Hello,

I am working on a design involving the THS8135 to drive an analog VGA output signal from a BT.656 input (1x10 bit input). My application requires a negative sync on green. The datasheet describes that the component "provides additional dc bias on R, G, B, and Y channels to provide headroom for negative sync insertion". Do I need to provide a negative supply to Vref (or another pin) to have the analog output drive to -300 mV, or do I need an external DC bias circuit to adjust the voltage such that 0 V is -300 mV? If so, do you have an example design? I looked at the THS8135EVM schematic, but that board did not support a negative sync.

Thanks for your time.

  • 0
    TI__Mastermind 43875 points

    The negative reference is relative to the blanking level and not actually a negative voltage with respect to ground.

    If you are using sync on green then the green blank level will be raise such that the sync can be inserted, and it will be negative with respect to the green blanking level.

    The receiving monitor will either AC couple or AC clamp the signal such that it can remove the sync and dc offset the green channel back to a zero reference (or whatever level the receiver needs)

    If you look at figure 1 in the datasheet you will notive that the right most measurements indicate the actual output voltages for a tri-level sync. Bi-level syncs simply don't have the second, positive going part of the sync pulse.

    BR,

    Steve

  • 0 in reply to Steve Clynes
    Prodigy 120 points

    Thanks for the quick response.

    It sounds like even though the analog output from the device does not comply with VGA signal levels, the receiving display will still be able to interpret the signal correctly?

    Does the transmitter side need to provide any AC coupling? The reference design does not show any.

    Any recommended buffer solution to protect the THS8135? Again, the reference design does not use a buffer between the device and the VGA connector.

    Thanks again for your help.

  • 0 in reply to Noah Root
    TI__Mastermind 43875 points

    Whilst technically not VESA compliant the most critical part of the VESA signaling is the deltas, not the absolute levels since pretty much all systems ultimately AC couple to the inputs. DC coupling is just not good for long cables

    You can simply add a large series capacitor (47uF+) to AC couple the outputs just before the connector. This is often not implemented since all receivers will AC couple their inputs since they need to do this anyhow in order to do DC restoration by clamping the input signals.

    Not sure what protection you are referring to? Do you mean for ESD or output short circuit?

    TI does have a number of analog VGA buffers, e.g. http://www.ti.com/lsds/ti/amplifiers-linear/fixed-gain-products.page?paramCriteria=no#p480=3 depending on your specific requirements.

    BR,

    Steve