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THS8136 expected behavior

Ted Pryor
Prodigy 70 points

Other Parts Discussed in Thread: THS8136

The COMP pin on our THS8136 was mistakenly attached to GND instead of AVDD. All of the other pins are connected as shown in the data sheet. Now we are only getting approximately 2V on the DAC outputs. Is this the expected behavior in response to this mis-connection?

  • 0
    TI__Expert 6120 points

    Ted,

    The COMP pin is not supposed to be connected to either GND or AVDD.  It's supposed to be bypassed to AVDD using a 0.1uF capacitor.

    You're comment that the DAC outputs are only 2V implies that you were expecting more than this.  However, the output voltage compliance range of the DACs is only 1.2V.

    Are you using a doubly terminated 75-ohm load (i.e. 37.5 ohms)?

    Do you have a 3.8-kohm resistor connected between the FSADJ pin and GND?

    Regards,

    Jim

  • 0 in reply to James Nave
    Prodigy 70 points

    Jim,

    The DAC is configured for RGB with sync insertion.

    The COMP pin is bypassed to GND (this is by mistake) using a 0.1uF capacitor.

    FSADJ is connected to ground through a 3.8 KOhm resistor.

    There are 75 Ohm teminations to ground at the DAC outputs, but we are not driving a load; only looking at the outputs with a scope.

    We are driving the DAC with R, G, B, Blank, and Sync. We expected to see some kind of activity on the outputs. However, all we have are DC signals at appx. 2V.

    I am wondering if the mistake on the COMP connection would cause the outputs to remain constant even though the inputs are being driven with video signals?

     

    Thanks,

    Ted

  • 0 in reply to Ted Pryor
    TI__Expert 6120 points

    Hello Ted,

    I doubt that bypassing the COMP pin to GND rather than AVDD would cause the output to be fixed at 2V.

    This is most likely being caused by using a 75-ohm load rather than a 37.5-ohm load.

    Regards,

    Jim

  • 0 in reply to James Nave
    TI__Genius 16810 points

    Ted,

    W are not sure how the DACs will behave with COMP incorrectly connected.

    You may want to also check /Sync, /Blank, and Sync-T logic levels during active video.  These signals must be high during active video to pass active RGB signals.

    Regards,

    Larry

  • 0 in reply to Larry Taylor
    Prodigy 70 points

    Larry,

    We are getting activity on the digital inputs; R, G, B, /Sync, and /Blank. Sync-T is connected to ground because the chip is configured for RGB Sync Insertion. All of the inputs look to have proper video signals on them.

    After further reading, I realized that the COMP circuit watches the ripple on the AVDD and adjusts the DAC output accordingly. It doesn't seem like it should cause a steady HIGH on the R, G,  and B DAC outputs. I am confident the steady high measures 2V because the system is not properly terminated. The measurement is taken at the chip without a monitor connected.

    Thanks for the suggestions,

    Ted