This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

PGA309 offset adjustment capability

Other Parts Discussed in Thread: PGA309

In looking at the PGA309 user guide, table 1.1, the Zero Offset adjustment capability is stated as +/-200%FS (coarse and fine combined).  I assume FS means the full scale output of the sensor, in mV.  However, it seems like the offset adjustment should be more.  +/- 48mV coarse offset, and an additional fine offset adjust from the ZeroDAC of up to 4096mV/input-stage-gain (RTI).   Is there a limitation I'm missing?  Thanks for your help.

  • Hello David,

    It really makes more sense to think of the offset adjustment capability in absolute terms. The PGA309's coarse offset correction can provide a maximum of ±60mV, referred to the input of the front-end PGA. While the Zero DAC can provide voltage up to the level of VREF, it is applied at the output of the front-end PGA and should only be used for offset drift correction. The most common way the Zero DAC is configured is usually to its mid-scale output (VREF/2), and then adjusted from there to provide the maximum possible temperature correction in either direction.

    In short, you should ensure that any initial offset in your sensor can be canceled by the PGA309 coarse offset. The range of this offset is given by the equation:

    VCOARSE_OFFSET_MAX = ± 14*(VREF/1200)

    For internal VREF levels of 2.5V and 4.096V, this range becomes ± 29mV and ± 48mV, respectively. The maximum external VREF which can be applied is 5V, giving a range of ±58mV.

    Best regards,

    Ian Williams
    Applications Engineer
    Precision Analog - Op Amps 

  • Thanks, Ian.  That's exactly what I thought.  Table 1.1 in the User Guide is a little misleading on Zero offset. (+/- 200%).  

  • Hi Ian, I have a follow-up question on offset adjustment.  It may seem small, but I have a failure analysis that would benefit from the answer to:

    Is the PGA309 coarse offset resolution

            (A)  VREF  X  0.00085 ?  

      Or   (B)  VREF / 1200 ? 

    Both are mentioned in the Users Guide.  The difference is 2%, which is more than just rounding error.

    Perhaps...?

           (A) applies to VREF = +5V and (B) applies to VREF < +5V?

      Or (A) is for external VREF and (B) is for internal VREF?

    Thank you,

    Dave Kerr

  • Hi Dave,
    I am looking into this to get a definitive answer. The disagreement also continues to the datasheet vs. the user's guide. The datasheet lists the .00085 x Vref figure. Regardless which number is correct, it shouldn't be dependent on vref source or voltage, I'd expect it to be dependent entirely on the coarse offset DAC itself.