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ADS1115: ADS1115

Santiago Beron
Prodigy 20 points
Part Number: ADS1115

I am trying to measure a 70 VDC (very limited power source. Aprox 3 mA) with very high precision. I used a resistor voltage divider (8.2 MOhms and 536KOhms) a buffer with an op/amp (gain 1) and an RC filer as front end for the ADS1115, but I get a very low frequency (20 s) noise on top of the DC sample signal measured that is not on the 70 VDC input signal. I tried changing the cut off frequency of the RC filter before the ADS1115 but due to the very lo frequency, the overall response of the circuits becomes several minutes. This prevents using the measured signal to control the generation of the 70 VDC signal

Any ideas?

  • 0
    TI__Guru* 98025 points

    Santiago,

    Thanks for your question on the ADS1115. There isn't anything that I know of that could cause a noise that has a 20s period in the ADS1115. The ADC does have an internal oscillator, but it runs at 250kHz.

    Are you able to post an excel spreadsheet of the raw data coming from the device (maybe in decimal or hex)? It might help just to see the raw data and see what this noise looks like. What are your configuration registers set to? And are you using continuous conversion mode or single-shot conversions? Have you checked the analog inputs with an oscilloscope? I want to make sure this noise isn't actual noise a being measured by the ADC.

    Regardless, look over my comments and let me know.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    Thank you Joseph,

    I was able to remove the noise by adding a capacitor in parallel with one 536K resistor of the voltage divider. Still not sure where the noise was coming from but I don;t think it was coming from the ADS1115

    Regards

    Santiago

  • 0 in reply to Santiago Beron
    TI__Guru* 98025 points

    Santiago,


    I'm glad you were able to fix the problem with some basic low pass filtering. I'd note that your voltage divider (536kΩ and 8.2MΩ) has a rather high output impedance. If you look in the ADS1115 datasheet on page 16, you'll see that the equivalent input impedance to the ADC is structured like the following:

    And the equivalent input impedance has values that depend on the FSR setting, as shown in the electrical characteristics curves:

    If you're measuring the 536kΩ resistor, you may be loading it with some parallel impedance. This may change your measurement a little. Again, I'm glad that you're able to make clean measurements.


    Joseph Wu