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ADS8861: SCLK reset and input capacitance

Oba
Genius 17325 points
Part Number: ADS8861
Other Parts Discussed in Thread: ADS8881

Hello,

Our customer has two questions about ADS8861/81.

1.
Is there any way to reset internal clock count of SCLK?
I mean that is it possible to use ADS8881 as 16bit ADC?

2.
Could you tell me input capacitance value of digital input pins?

 
Regards,
Oba

  • 0
    TI__Mastermind 46560 points

    Hello Oba-san,

    1.  Yes, you can only send 16 SCLK's to transfer the 16 MSB's and not use the 2 lower LSB's.  This will not work in daisy-chain mode, but will work in the other modes.

    2.  The IBIS model includes the input capacitance for the digital pins.  http://www.ti.com/lit/zip/sbam172  The package capacitance is around 0.13pF, and the die capacitance is around 0.38pF.

    Regards,
    Keith Nicholas
    Texas Instruments
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    TI__Genius 17325 points

    Hello Keith,

    Thanks for your reply.
    Regarding capacitance, I would like to know how we can read from IBIS file.

    For example, the below is from the IBIS file for DRC package.
    I found the package capacitance is around 0.13pF from it.
    But I couldn't found 0.38pF for die capacitance. Is it pin capacitance?
    From the ibis file, it look like around 0.13pF. Where does your 0.38pF come from?
    And is it OK to simply think input pin capacitance is the package cap + pin(die?) cap?

    [Component]    ads8881drc   
    [Manufacturer] TI
    |
    [Package]      |10 pin SON - DRC designator
    | variable   typ     min           max
    R_pkg  4.01071E-02   3.98300E-02   4.07400E-02
    L_pkg  6.73961E-01nH 6.67530E-01nH 6.88940E-01nH
    C_pkg  1.31966E-01pF 1.20970E-01pF 1.37080E-01pF
    |  
    |-------------------------PIN------------------------------------------------
    [Pin] signal_name  model_name  R_pin  L_pin C_pin
         
    1   REFIN   TERM       0.03989  0.67676nH  0.12144pF
    2   PVA     POWER      0.04011  0.67814nH  0.13629pF
    3   PIN     TERM       0.04003  0.67638nH  0.13635pF
    4   MIN     TERM       0.04074  0.68894nH  0.13708pF
    5   GND     GND        0.03982  0.67551nH  0.12111pF
    6   CONVST  IO_BUF     0.03983  0.66753nH  0.12097pF
    7   SDO     SDO        0.04003  0.66857nH  0.13582pF
    8   SCLK    IO_BUFDUAL 0.04021  0.67121nH  0.13614pF
    9   SDI     IO_BUF     0.04002  0.66834nH  0.13596pF
    10  =VBD    POWER      0.03983  0.66749nH  0.12101pF

    Regards,
    Oba

  • 0 in reply to Oba
    TI__Mastermind 46560 points

    Hi Oba-san,

    The package capacitance is the approximate value for each of the pins; the PIN value is the more precise value for a specific pin including the package, lead frame, and bond wires.

    If you scroll down through the IBIS model, you can find the details for the actual die capacitance (C_Comp), and this is a range that depends on process variation, temperature and IO voltage level.

    For 1.8V operation, here is the approximate range of values:

    ***************************************************************************
    | 1.8 V IO_BUF Model
    |***************************************************************************
    |
    [Model] IO_BUF_1p8
    |
    Model_type Input
    Vinl = 0.54
    Vinh = 1.26
    |
    |
    | typ min max
    | (nom PVT) (Fast PVT) (slow PVT)
    C_comp 3.856E-13 3.091E-13 4.614E-13


    Take a look at this application note that shows how to use R, L, and C values for a full simulation.

    http://www.ti.com/lit/an/slyt390/slyt390.pdf

    Regards,
    Keith