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Sigma-Delta non-linearity at end points of scale (ADS1015)

Aaron Ridinger
Intellectual 490 points
Other Parts Discussed in Thread: ADS1015

Hello,

I have two analog inputs, one from a light level sensor ranging from 3mV to 3V, and the other a conditioned and amplified signal from an RTD ranging from ~1.5 to 3V.  I will only need to monitor these sensors every second or so, so speed is not an issue. Vcc=+3.3V

I am trying to determine if a sigma-delta or SAR ADC is more suited to this application.  From my reading it seems that either would be fine, except for one point I read here (http://www.cypress.com/?rID=43323).  This article states that sigma-delta converters have non-linearity at the end-points of the scale and should not be used in applications that require linearity rail-to-rail.  Would this be a problem with the ADS1015, and specifically would it affect my application?

Thanks in advance,

Aaron

  • 0
    TI__Guru**** 181021 points

    Hi Aaron!

    Welcome to our forum!  This sounds like an interesting application!  Can you provide a little more detail?  It sounds like a two channel device with an internal PGA may be appropriate here, which would lean more towards the delta-sigma approach.  Did you note the (2) comment beside the INL Test Conditions specification for the ADS1015?  The 0.5LSB MAX is based on a best fit curve at 99% of the full scale input range.  We'll take a look at the document you sited and provide some additional feedback ASAP. 

  • 0 in reply to Tom Hendrick
    Intellectual 490 points

    Hi Tom,

    Thanks for the response.  The RTD is built into a heated window and will be used for feedback for monitoring window temperature.  I'm using a 1mA current source to excite the RTD and then filtering and amplifying it using an op-amp with a gain of 21.  However, using the internal PGA (set to gain of 16) may work better than the discrete filtering and amplification.  The light sensor output is amplified at the sensor, so an internal PGA could be set to 1 when getting data from that channel.

    I did see the INL Test Conditions about the non-linearity 0.5LSB max, but I missed the note about 99% of full scale.  That sounds like non-linearity should not be an issue then!

    Thanks,

    Aaron

  • 0 in reply to Aaron Ridinger
    TI__Guru**** 181021 points

    Hi Aaron,

    I believe you'll be happy with the performance.  Give it a try and let us know what you see.

  • 0 in reply to Tom Hendrick
    Intellectual 490 points

    Thanks Tom,

    I'll design it in and see how it works.

    Aaron