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ADS6442: What range of capacitances can be added to pin # 5 (CAP)?

Pankaj Kataria
Prodigy 100 points
Part Number: ADS6442

I have designed in TI’s ADC ADS6442_RGC_64 in our prototypes currently being manufactured.

Per TI’s specification I am using a 2nF capacitor on pin 5 (CAP).

I picked a 2nF/50Vdc, 10%, 0402, X7R. The layout is already done and PWB is ordered. The issue is that this capacitor is extremely unusual and difficult to procure, so I would like to change it. We already have access to the following 0402 capacitors at manufacturing site and wondering if anyone of these would work

  1. 1uF/10%, 25V, X5R, 0402

  2. 1.2nF, 5%, 50Vdc, NPO, 0402

  3. 0.1uF, 10%, 25V, X7R, 0402

 

Please let me know ASAP so that I can get these prototype ASSYs made?

Please note that I have pin 6 (RESET) connected to ground via a 2K resistor as we have serial communication mode which will be reset via software.

Thanks in advance.

  • 0
    TI__Mastermind 29220 points

    Hi,

    There is not a lot of explanation in the datasheet for that pin, but from the block diagram on page 2 of the datasheet I can see that the CAP pin goes straight into the Phase Locked Loop for the device.  That tells me that this pin is for the loop filter capacitor for the PLL.    The value of the cap on this pin should be relatively close to the specified 2nF.   You would be able to go to the nearest standard value for the capacitor and not have to have exactly 2nF.  For example, 2.2nF would be a standard value.    Changing this loop filter component too much would change the bandwidth of the loop filter for the PLL and change its behavior somewhat, possibly taking longer for the PLL to power up to stable operation if the filter cap is much too large, or possibly have more jitter in the clocking for the serialized data outputs.   Usually the loop filter value is not critical, so the 1.2nF component that you have available might work.  I do not have characterization data to support the choice of 1.2nF, so I would be more comfortable with a standard 2.2nF value.

    Regards,

    Richard P.

  • 0 in reply to Richard Prentice
    Prodigy 100 points
    Thanks for the details Richard. Having a 2.2nF makes sense then.