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VSP5612: ADC clipping at 0xFBxx with APG=45, DPG=0

Daisuke Mori
Prodigy 10 points
Part Number: VSP5612

Tool/software:

Hello,

I am evaluating the VSP5612 with a CCD sensor.
With the gain settings of APG=45 and DPG=0, the output values only range from 0x0000 to 0xFBxx.
Even after changing APG and OFDAC, the clipping at 0xFBxx remains unchanged.

Since 255/251 ≈ 1.016, I tried increasing DPG_1 slightly, from ×1.000 (0) to about ×1.016 (code 4–5).
With this adjustment, the output extends up to 0xFFFF as expected. From the block diagram, this behavior suggests that the clipping originates in the ADC stage.

Using DPG as a workaround is acceptable in practice, but I am concerned that the root cause is unclear. The datasheet does not mention such a limitation in the ADC.
Am I overlooking something? Could you please provide any advice or clarification?

Thank you very much for your support.

Best regards,
Mori

  • 0
    TI__Expert 4920 points

    Hi Mori, 

    Sorry for the delay. 

    Do you observe this phenomenon on all the devices or only on some of the devices? 

    If you observe in all the devices, is the saturating code same (0xFBxx) for each device or keeps changing from device to device or run to run? 

    Thanks, 

    Karthik N

  • 0 in reply to Karthik Nagabhushana
    Prodigy 10 points

    Hi Karthik,

    (1)  Is this phenomenon observed on all the devices?
    → YES

    (2) Is the saturating code the same?
    → If the APG value is the same, there is no device-to-device variation.
    We have confirmed that changing the APG value causes the saturating code to change.

    Best regards,
    Mori

  • 0 in reply to Daisuke Mori
    TI__Expert 4920 points

    Hi Mori, 

    (1) Okay

    (2) I did not follow. In your first message, I thought you said changing APG was not changing the saturation code, but changing DPG eventually changed the saturation code to 0xFFFF. Is my understanding correct?

    Btw, what is the input when you try to saturate the AFE? 

    Thanks, 

    Karthik N 

  • 0 in reply to Karthik Nagabhushana
    Prodigy 10 points

    Hi Karthik,

    (2) Regarding the saturation code vs. APG:
    Apologies for the confusion. In my first message I mentioned that changing APG does not affect the saturation code, but after further testing I found that is not the case.
    The measured results of how the saturation code depends on APG are as follows:

    (3) Regarding the input to the AFE:
    The AFE is connected to a CCD linear sensor (TCD1209DG), and we are illuminating it with a lamp.

    Best regards,
    Mori

  • 0 in reply to Daisuke Mori
    TI__Expert 4920 points

    Hi Mori, 

    (2) In the below table, for APG = 0x14, the max code of dec 243 is saturation voltage or just max voltage? Can you share digital output curve for this use case? 

    This is not expected. I am suspecting that TCD1209DG output is saturated.

    Can you double the exposure/light power and see if there is any change in VSP digital output? If there is no change in VSP digital output, then it would indicate that the CCD sensor is saturated. 

    (3) Okay. 

    Thanks, 

    Karthik N