Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

  • State TI Thinks Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 2 answers
  • Subscribers 45 subscribers
  • Views 2200 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

How to mitigate offset error from ADS1115 at 16X gain?

Ronnen Levinson
Prodigy 30 points
Other Parts Discussed in Thread: ADS1115

Hi.

I would like to use an ADS1115 connected to a BeagleBone Black to measure the potential difference (PD) output of circa-2011 Kipp-Zonen SP Lite2 silicon pyranometer. The sensor has an output range of approximately 0 - 10 mV, and a sensitivity of about 10 μV per W/m². The ADS1115 is operated at 5V VCC.

When making a single-ended measurement with the ADS1115 at 16X (FS = 256 mV) and 8 SPS, I get an offset error of about -0.3 mV (about -30 W/m²) with respect to a high-end Fluke multimeter that has μV accuracy. I believe the multimeter, rather than the ADS1115, because in darkness I read essentially zero from the multimeter, and about -0.3 mV (-30 W/m²) from the ADS1115.

What steps can I take to reduce the offset error observed with the ADS1115? For example, would switching to differential measurement help?

Yours truly,

Ronnen.

  • 0
    TI__Intellectual 1610 points

    Hello Ronnen,

    Thank you for your inquiry. The specification in datasheet is with shorted inputs, right at the input pins.

    There can be several reasons that can contribute to offset error. I have few concerns here:

    1. Are you using the internal GND option for single-ended measurements? If yes, how do you short the single-ended channel to GND?

    I would try to short the single-ended channel pin to GND pin of ADS1115 directly and check if you see any improvements. There could be a mismatch between the two GND voltages.

    2. Can you short the differential inputs together right at the input pins, select that setting and see if you see any better results? Just keep in mind that a mismatch in the input pair routing could cause different IR drop through the traces and could contribute to an offset error

    3. Have you tried shorting the Fluke multimeter with probes included and see that the offset is within few uVs? Sometimes, the IR drop through the probes could contribute to some voltage differential.

    Feel free to share your schematic with us.

    Thanks,

    Krunal

  • 0 in reply to Krunal Maniar
    Prodigy 30 points

    Thanks, Krunal.

    Are you suggesting that I make a series of PD measurements of the form

    1. single-ended AIN0 vs GND, with AIN1 shorted to GND;
    2. differential AIN1 vs AIN0, with AIN1 shorted to AIN0; and
    3. multimeter, probes shorted?

    Yours truly,

    Ronnen.

  • 0 in reply to Ronnen Levinson
    TI__Intellectual 1610 points
    Ronnen,

    Yes, you can try:

    1. AIN0 pin shorted to GND pin (select the Input Multiplexer setting 100 : AINP = AIN0 and AINN = GND)
    2. AIN0 pin shorted to AIN1 pin (select the default Input Multiplexer setting 000 : AINP = AIN0 and AINN = AIN1)
    3. Yes, short the probes and check what the Fluke multimeter measures

    Regards,
    Krunal
  • 0 in reply to Krunal Maniar
    Prodigy 30 points
    Hi Krunal.

    In configurations 1 and 2, I joined GND, AIN2, and AIN3 over short lengths (10 - 20 cm) of 24 AWG stranded wire to a common ground with a WAGO 222 five-wire connector. I set PGA=16X, SPS=8.

    (1) Single-ended measurements
    AIN2 to GND: -0.195 mV
    AIN3 to GND: -0.195 mV

    (2) Differential measurements
    AIN3 (positive) to AIN2 (negative), -0.313 mV
    AIN2 (positive) to AIN3 (negative), +0.0078 mV (note: 1 LSB)

    Results in configurations 1 and 2 were repeatable to within 1 LSB (here, 256 mV/2^15 = 0.0078 mV).

    (3) Shorting a pair of Pomona Electronics Minigrabber clips
    Fluke 289 multimeter: 0.002 to 0.007 mV

    Yours truly,

    Ronnen.