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ADS7809 - REF pin needs 2.2 uF cap - Why?

Kenneth Aquino
Expert 1300 points
Other Parts Discussed in Thread: ADS7809, ADS8509

I know the data sheet says that the REF pin on the ADS7809 needs to be bypassed, and I believe it having seen noise problems with a similar converter when the REF pin was not bypassed with a fairly large capacitor.  Why is this?

  • 0
    TI__Expert 6925 points

    Hi Kenneth,

    In general, a large capacitor (>1uF) must be placed at the reference input of analog-to-digital converters, and it has two functions:
    1) to limit noise bandwidth of reference and/or buffer
    2) to keep the reference voltage very stable during conversion time.

    Since the ADS7809 has an internal buffer and reference, two 2.2uF capacitors are needed to limit noise bandwidth on both of them.  In the case of the reference noise bandwidth, the 2.2uF capacitor at the REF pins limits reference noise bandwidth to about 20Hz.

    Regards,
    Rafael

  • 0 in reply to Rafael Ordonez
    Expert 1300 points

    Hi Rafael,

    Are dynamic currents pulled from the reference circuit during each conversion due to the switched capacitor architecture of this converter?

    Regards,

    Ken

  • 0 in reply to Kenneth Aquino
    TI__Guru**** 168605 points

    Hi Ken,

    Yes, the reference is switched in/out with each bit decision that the ADS7809 makes.  Can you tell us anything about your application?  The ADS7809 has been listed as NRND (not recommended for new designs).  The ADS8509 is pin-compatible to the ADS7809 and has the same features at a lower price.  If this is a new project for you, I would suggest that you look into using the ADS8509 instead.

  • 0 in reply to Tom Hendrick
    Expert 1300 points

    Hi Tom,

    We're using the ADS7809 in a space application (equipment is mounted on a spacecraft) and the ADS7809 has flight heritage, that's why we like it.

    Ken

  • 0 in reply to Kenneth Aquino
    Intellectual 280 points

    Also,

    How does the ESR of the recommended 2.2 uF tantalum capacitor connected from the  the "CAP" pin to ground effect the performance? I see curves of SINAD and THD vs cap ESR in the ADS8509 (the recommended replacement for the ADS7809).  We have a problem wherein we see 6 MHz oscillation on the input pin to an ADS7809.  We changed the capacitor connected from the "CAP" pin to ground and the oscillation went away.  Does the ESR of the capacitor connected from "CAP" pin to ground affect the stability of the input?

    Ken

  • 0 in reply to Kenneth Aquino1
    Intellectual 280 points

    Also,

    Why is a tantalum capacitor recommended for the capacitor between "CAP" and ground?  Tantalum's have more ESR than ceramics, and a ceramic is (nowadays anyway) available in 2.2 uF.   Is there a minimum amount of ESR that is required to prevent the internal buffer amplifier shown below (from the ADS8509 data sheet) from oscillating?  If so, what is that minimum ESR value?

    Ken