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ADS62P24: ADS62P24 Vcm and Reference accuracy

chris hamilton
Prodigy 30 points
Part Number: ADS62P24
Other Parts Discussed in Thread: ADS62P22

I have a design where I'm seeing two 'bins' of ADS62P24 behavior regarding Vcm.  These 'bins' may be different lots of the ADS62P24, that's not clear to me yet.

One group puts out about ~1.54V on Vcm and has a measured ADC gain error of about -1%.  

A second group puts out ~1.63V on Vcm and has a measured ADC gain error of about -6%.  

Ok...so my questions:

1) What is the expected range of Vcm.  The datasheet just says 1.5V typical. Does 1.63 represent an abnormality?
2) Does Vcm correlate directly back to the internal reference?  My data shows such a correlation but the datasheet isn't entirely clear.

  • 0
    TI__Guru* 91947 points

    Chris,

    We are looking into this.

    Regards,

    Jim

  • 0
    TI__Guru* 91947 points

    Chris,

    The VCM is created by the internal reference (see figure 88). The typical values of this output can be 1.4V - 1.6V, so a value of 1.63 could be expected. I do not know what the min and max values could be for this but I am looking into it.

    Regards,

    Jim

  • 0 in reply to jim s
    Prodigy 30 points

    Thanks for the response.  Yeah I've stared at the diagram....created by how?  There are two lines coming out of the box named simply 'internal reference'.  It would make sense if those two lines were THE reference voltage but that wouldn't be consistent with the rest of the diagram.  The VCM is gained by the 1k/4k network which implies Vcm is derived from 1.2V.  But we know from Equation 2 that INTREF should be 1.5. for the nominal 2Vp-p range.

    Regardless I have a gain problem and understand Vcm accuracy wouldn't normally matter.  The reason I'm asking in this case is that it's the only thing derived from the Internal Reference that I have access to on the pins.

    So bottom line: are you telling me that if one chip has Vcm of 1.62 and another has a Vcm of 1.54 that their internal references must be different by the same ~5%?  That does explain what I'm seeing.

    Other notes:

    1) I disconnected the Vcm pin from the rest of the circuit wondering if it was being loaded and impacting the internal reference.  That had zero impact on my end-application gain error

    2) I'm actually using ADS62P22, not 24 (assuming it doesn't matter)

  • 0 in reply to chris hamilton
    Prodigy 30 points

    I can add that we re-worked a board to change just the ADC and we saw a full 5% shift in amplitude accuracy proving again we have parts with 5% variation in gain accuracy between them.

    Still looking for guidance on how we should treat these parts.  Are they compromised or not? 

    The full spec is +3%/-3% meaning 5% can fit within it...however it seems unlikely we have parts that happen to be at the extreme end of the spectrum.  I can provide the lot codes if its helpful.