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ADS1255 internal buffer

pooja trivedi
Intellectual 455 points
Other Parts Discussed in Thread: ADS1255, ADS1256

Hii,

I am using ADS1255 ADC. the problem is that when i give input to AIN0 when buffer is on it will give reading upto 2.1V.but when buffer is off input range is 3.0V. means when i off the buffer ADC count is decreased for same input.

my reference is 1.2 V. and i use ADC in 18 bits but it should give count to only 244k instead of 262K.

Please provide suggestions.

  • 0
    TI__Guru* 98145 points
    Pooja,


    The input buffer has an input range that does not extend the full supply range from AGND to AVDD. If you look at page 3 of the ADS1255 datasheet, the input range with the buffer on has a range from AGND to AVDD-2.0V. This likely is limiting the input range of your measurement.

    Check the input for AIN0 and AIN1 and verify their voltages. Compare them against the input range of the buffer. If this turns out to be the problem, post back letting us know.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 455 points

    Hi joseph,

    Thank you for your reply. I think the issue is related noise as i am using circuit made on general purpose board.

    if possible can you proivde .brd file for layout for ADS1256 - EVM?

    Thanks in aadvance.

  • 0 in reply to pooja trivedi
    TI__Guru* 98145 points
    Pooja,


    This question came came up earlier, and we don't have the ADS1256EVM in a .brd file because it was not made in Eagle or in Allegro. You can see my response in the link below:

    e2e.ti.com/.../437038


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 455 points
    thank you joseph,

    One thing i want to ask that in ADS1256 is settling time is related to PGA?
    If we change PGA is there any change in settling time?
  • 0 in reply to pooja trivedi
    TI__Guru* 98145 points

    Pooja,


    The settling time will be based on the digital filter. The PGA value does not affect the settling time in the ADC. 

    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 455 points
    Hii Joseph,

    i am using ADS1256evm now and i provide the +5.0V supply from Battery and reference is generated from this +5.0 volt with LM4041-1.2 V.
    I also provide input to the ADC thru battery.
    with this i only get 16 bits stable at 25 SPS fron 24 bit. What shold be the issue?
  • 0 in reply to pooja trivedi
    TI__Mastermind 44720 points
    Hi Pooja,

    First test the noise performance with the ADC inputs shorted. You can do this on the ADS1256EVM by throwing switching S1 to the right. This will show you the noise performance of the ADC alone (without the input and reference noise).

    Once you've verified the ADC's noise performance, then move on to applying a low-noise input signal.

    NOTE: I've found battery inputs to be noisy. On a 12-bit ADC you won't see the battery's noise, but on a 24-bit ADC you certainly will. Also, a floating battery connection could be violating the ADC's input range. The battery voltage needs to be referenced to AGND (or another mid-supply voltage).

    Best Regards,
    Chris
  • 0 in reply to Christopher Hall
    Intellectual 455 points
    Hi Chris,

    Thank you for your reply. When i short the ADC inputs i get the stability of 17 bits at 1000 sps. but when i applied input from DC source i get only 14 bits. What should i do to reduce the input noise. I put RC filter at input (R = 10K and C = 0.1uF).

    Please provide suggestion.
  • 0 in reply to pooja trivedi
    TI__Mastermind 44720 points
    Hi Pooja,

    An RC filter at the ADC's input only helps when the cutoff frequency is very low or the RC values are very large. This is because the digital filter of the ADC provides the majority of noise filtering, when set to a low data rate. However, a large RC filter increases the overall circuit settling time, so usually the RC filter only serves as an anti-aliasing filter, not as an input noise filter.

    Slowing down the ADC's data rate will decrease the digital filter's bandwidth, which will improve the noise performance. This is a normal trade-off with delta-sigma ADC's.

    In your case, the input signal itself may be noisy or it may not be stable (a drifting input signal may also make the noise appear larger. Either you'll need to use a low-noise input signal source, or capture a shorter set of data (less drift will be seen in a shorter time frame).

    Additionally, noise will also increase as your input signal increases. This is because reference noise tends to have a more significant effect for larger inputs. If you try changing the input voltage and notice that noise scales accordingly, then it may be reference noise you're observing.

    I hope that gives you some suggestions you can try to understand the type of noise your seeing.

    Best Regards,
    Chris