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THS7316: Attenuation on VGA signals using the THS7316

DHAIVAT JANI
Prodigy 230 points
Part Number: THS7316

Hi there,

I am trying to achieve a VGA splitter (using THS7316) on our baseboard. We are using this for a VGA signal with a fixed resolution of 1024 x 768 @ 60Hz and connect to a display panel and a monitor. 

Attached (VGA-2.png) is the circuitry that worked last time with a COM Express module. However, as the module has now changed the input to these VGA buffers is now coming from a Chrontel IC CH7517.

I have verified that VGA signals coming out of the Chrontel IC CH7517 is 0.7Vp-p using oscilloscope and VESA compliant when not connected to the VGA buffer THS7316. However, as soon as the buffers are connected there is a significant attenuation of the VGA signals at the input of the VGA buffer THS7316. The output of the VGA buffer THS7316 is also the attenuated signal with the DC bias.

 

Whilst I have now also tried the circuit by removing the AC coupling (capacitors) and employed a DC coupling as shown in the circuit (VGA-1.png) below, I did not see any improvement.

Even this results in significant attenuation of the VGA signals at the input of the VGA buffer THS7316. The output of the VGA buffer THS7316 is also the attenuated signal with the DC bias. Can you please suggest what I must do to create a VGA splitter circuit. Please note that the 150ohm resistor pull down to ground at the input of the buffer s a requirement of the COM module as there is another 150 ohm resistor on the COM module (at output of the Chrontel IC CH7517). This makes it a 75 ohm.

I have also tried using just one VGA buffer THS7316 to drive both the display panel & monitor and still it has the same attenuation with dc bias at the output. Can you please suggest a fix?

  • 0
    TI__Mastermind 28695 points

    Hello Dhaivat,

      Thank you for the images and the information. Would you be able to share scope shots of the attenuation? If not, how much attenuation is there at the inputs of the THS7316 vs directly at the output of the module?

    Thank you,
    Sima

  • 0 in reply to Sima Jalaleddine
    Prodigy 230 points

    Hi Sima,

    Please find attached the traces at input & output of the buffer THS7316. Please note that there is also a 150 ohm pull down to ground at the input of the buffer which along with another 150 ohm pull down to ground on the COM module results in 75 ohm at the input of the buffer IC THS7316.

  • 0 in reply to DHAIVAT JANI
    Prodigy 230 points

    Hi Sima,

    Also find attached trace of the VGA signals originating from the COM module (at output of the Chrontel IC CH7517) when no buffer ICs THS7316 are connected and the VGA signals are routed directly to the monitors (shorted through).

  • 0 in reply to DHAIVAT JANI
    Prodigy 230 points

    Hi Sima,

    The strange behaviour of the IC THS7316 is that when I reduced the resistor at the input of the buffer from 150 ohm to 51 ohms (which in parallel with 150 ohm on the COM module results in roughly 38 ohm equivalent resistance.) The buffer circuits start to function.

    See attached signals below

    Output of the buffer IC (DC coupling circuit)  

    Output of the buffer IC (AC coupling circuit)  

    Can you please therefore suggest what sort of input impedance is required for IC THS7316 to function properly? The reason is I dont want to be changing my VGA driver circuit for every different COM Express module. 

  • 0 in reply to DHAIVAT JANI
    TI__Mastermind 28695 points

    Hello Dhaivat,

      Thank you for the all these scope shots. The THS7316 has an input resistance of 800kOhms, and the internal voltage shift wouldn't be affecting the input. I need to look into this issue a bit further. In the meantime, I was looking into the COM module, and it has a DAC which is based on a current source architecture. This would mean the configuration of the THS7316 should be DC coupled. The DAC within this module has connection detection to determine if it will go into power saving mode. Do you know anything about this feature, and how it works? It could be going into power saving mode in this situation. Would you be able to connect the output of the module to a 150Ohm load resistance compared to a 50Ohm load resistance (instead of the THS7316)?

    Thank you,
    Sima