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ADS7959's reference input

Pinar Kesik
Prodigy 20 points
Other Parts Discussed in Thread: REF5025, ADS7959, REF3325, REF3125

Hello everybody,

I have a question about ADS7959's reference input. In its datasheet, I couldn’t find a principal schematic of the Reference Input. It is offered to use REF5025 as a voltage reference but it is too precise for my design.

0.4% accuracy is enough for me. So, I think that using a voltage divider with 0,1% Resistors is sufficient to me. I wonder if reference input is really a buffered input with resistive appearance or a switched cap input. If it is resistive, I need to know input resistance and an indication of the possible variations of input resistance. On the other hand if it is capacitive, I need to know input capacitance and an indication of the possible variations of input capacitance.

Could you please support me about this issue?

Best regards,

Pinar

  • 0
    TI__Genius 9145 points
    Hello Pinar,

    The reference input to ADS7959 is not internally buffered. The external circuit will be presented a switched cap load.

    The worst case switch cap load is expected to be 8.2pF.

    If you plan to use resistive divider to generate 2.5V, I will recommend to buffer the same before connecting to REF input. Also have a decoupling cap on REF input. The buffer should be able to drive the decoupling cap.

    Since you are okay with lower precision, you can also consider other reference offerings from TI such as REF3325, REF3125 etc.

    Please check www.ti.com/.../series-voltage-reference-products.page

    Thanks & Regards,
    Shridhar.
  • 0 in reply to Shridhar%20More
    Prodigy 20 points

    Hello Shridhar,

    Thanks a lot for information.

    I'm also confused about one topic. In ADS7959's datasheet, there is an information about "Reference Resistance". Its value is given 100kohm (typ). 

    Could you please inform me about what Reference Resistance is? I thought it is used for current limiting, but I'm not sure.

    Best regards,

    Pinar

  • 0 in reply to Pinar Kesik
    TI__Genius 9145 points
    Hello Pinar,

    The switched capacitive load offers an impedance which can be represented as resistive load at particular frequency.

    This value varies with switching frequency. The value for ADS7959 ADC's capacitive load at 1MSPS operation is provided here for information.

    Thanks & Regards,
    Shridhar.