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PGA309: Deactivating Coarse Offset DAC Chopping - What are potential drawbacks?

franziskus
Expert 7498 points
Part Number: PGA309

Team,

my customer encounters noise issues in their pressure sensors using PGA309. This is mainly caused by the Coarse Offset DAC.
There are two main noise sources at 2kHz and 3.5kHz visible in the actual signal.

The Users Guide suggests to change the CLK_CFG Mode from "00" to "01". This leads to improved noise performance. The drawback however, is the Coarse Offset DAC is no longer temperature-stable anymore, but the effect should be calibrated out as mentioned in the picture below.

Are there any additional drawbacks that may not be described in the User Guide?

Thanks!

  • 0
    TI__Guru 54576 points

    Hi Franziskus,

    As the Customer has mentioned, the main effect is that the drift over temperature of the coarse offset DAC will be present.  As mention in the user guide, the DAC offset will be typically linear, and therefore the drift errors of the DAC can be calibrated out together with the offset of the bridge sensor.  In addition, please refer to the "PGA309 Noise Filtering" Application Report that discusses the noise effects of the Coarse offset Adjust DAC when chopping is enabled and disabled.

    http://www.ti.com/lit/an/sboa110a/sboa110a.pdf

    Please see the plot below of the noise density of the PGA309 when the Coarse DAC offset feature is used, with chopping enabled.  The plot shows chopping noise components at 3.5-kHz and 4-kHz; and a flat noise spectrum at low frequencies (no 1/f noise or flicker noise).   

    One advantage of using chopping is that the noise density of the amplifier looks completely flat at lower frequencies, in other words, there is no low frequency 1/f  noise or flicker noise present.  Most linear, non-chopping amplifiers/circuits present some amount of 1/f or flicker noise over frequency which could affect the effective resolution of the measurement.  In many cases, the bridge sensor signal of interest is low frequency, and therefore, in many cases the application allows the use of an external RC filter to reduce bandwidth, and reduce the noise components of the chopper in the spectrum.  Below is a plot of the noise spectrum with the Coarse DAC offset chopper enabled to maximum, with and without an external 100-Hz filter (Figure A-3 on the application report).

    Many users use chopping enabled and an external RC filter to reduce the noise components of the chopper. The filter is set to the smallest bandwidth that will work for the application.  If the Coarse DAC chopper is disabled, the drift of the DAC can be calibrated together with the offset/drift of the bridge sensor, but a trade off is that some amount of low frequency, 1/f noise may be present, as it is the case of most amplifiers/linear circuits.  

    Many Thanks,

    Best Regards,

    Luis

        

  • 0 in reply to Luis Chioye
    TI__Expert 7498 points
    Hi Luis,

    thank you very much for this detailed answer!

    Best regards
    Franziskus