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LM98640: Odd/Even Ping Pong Effect?

Stefan Hilbert
Prodigy 40 points

Part Number: LM98640

Hello,

we intent to use the LM98640 in CDS and Quad Lane Mode at 25 MHz Pixel Rate.

Like described in your data sheet in CDS Mode "the input sampling amplifier has two physical paths (odd/even) through which a particular pixel will be sampled. These two sampling paths are a requirement in the Correlated Double Sampling architecture."

OK - furthermore will be explained that:

"The LM98640QML will indicate (with the TXFRM signal) whether the pixel traveled the even path or the odd path. For all "Odd" pixels, the TXFRM signal is high for three TXCLK periods ..."

But here seems to be the problem - there's ONE TXFRM Signal only but TWO ADC output lanes.

For which of the two output lanes the TXFRM is valid in relation of ODD or EVEN ??? That is not clarified!

Based on this feature we got In front of the design procedure from our customer following questions (experience from another project):

Each time the camera system is initialized (power on) the 1st pixel through each of the two ADCs (in one package) can be randomly ODD or EVEN! This results in 4 possible configurations for the two ADCs: ODD/ODD, EVEN/EVEN, ODD/EVEN, EVEN/ODD.
It seems to there is no way to
1) force the two ADCs into a specific default configuration and
2) to detect with one TXFRM odd/even of two ADCs!
 
Thanks and hopefully waiting for an answer,
S. Hilbert

  • 0
    TI__Genius 14485 points

    Hello Stefan,

    I've contacted the applications engineer who supports this device.

  • 0
    TI__Expert 4940 points

    Hello,

    The two channels are in sync, both are ODD/ODD or EVEN/EVEN .

    After power on the part will come into a random state, first frame can be EVEN or ODD with same state for both channels.

    TXFRM timing info is valid for both channels:

    - 3 clock cycles TXFRM - both channels are EVEN

    - 5 clock cycles TXFRM - both channels are ODD

    What 640 type are you using?  LM98640(68VQFN plastic package) or LM98640QML( 68CFP ceramic package)

    In this case that's no difference between them  but other parameters may be different.

    Regards,

    Costin

     

  • 0 in reply to Costin Cazana
    Prodigy 40 points
    Hello Costin,

    thanks for your explanation.
    Hopefully you are right, currently we have no experience with the device, it was an information from our customer only.
    We intent to use the LM98640QML for space application.
    If we have made the first tests (we have a eval. board in our house) I'll come back to you!

    Best regards
    Stefan