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Original question:

ADS1115: Inaccurate differential Measurement above VDD/11

ADS1115: Inaccurate readings at high voltages

Rohit Mokkarala
Prodigy 10 points
Part Number: ADS1115

I'm using an ADS1115 in differential mode with a step down setup of 330K/3.3K Ohm resistors to measure the voltage of a supercapacitor. The range of the super capacitor is 0-360V, with the step down adjusting that to 0-3.6V. I have attached a schematic of the circuit below, but there are two issues with the voltage measurements. The raw voltage being read is negative, and the corrected voltage (multiplying the ads differential input by a constant) starts to show significant inaccuracies (>1V) at high voltages (>100V). Is there an issue with how the input pins are tied to ground? The green wire at the bottom at the - terminal of the breadboard is connected to ground of a separate power supply. The 6.0F capacitor is what's voltage is being measured.

  • 0
    TI__Mastermind 22950 points

    Hi Rohit,

    What is the reason for separating grounds? Are you just trying to isolate the high voltage GND with the low voltage side to reduce the risk of any high side ground noise affecting the low side GND? Additionally, are you measuring against theoretical values on the input? Have you measured the voltage at A1 to ensure that there isn't a voltage error at the input itself?

    It would also be helpful to know the device configuration. Are you able to provide that? For example, what data rate are you using? Are you using continuous conversion mode or single-shot mode?

    At first glance, the .47uF cap seems pretty large. As does the resistor to GND (3.3kohms). We typically recommend keeping that resistor to 1kohms or less due to a current divider formed at the input. Is it possible to reduce both the 3.3kohm resistor to 1kohms and the 0.47uF cap to 10nF or so and see if the measurements get any better?

    Regards,
    Aaron

  • 0 in reply to Aaron Estrada
    Prodigy 10 points

    Hi Aaron, thanks for the response.

    Yes, ground is separated to isolate noise which was an issue if not separated. To clarify, the ADS1115 measurements are being compared to a digital multimeter, which is where the discrepancy arises.

    The lowest capacitance capacitor I have access to is 470nF which alone did not improve the measurements. I can reduce the step down resistor setup to 82 kOhm/820 Ohm which is the closest to 1kOhm I currently have. As for the data rate, it is the default which is 128 SPS I believe. I am not sure how to check whether I'm using single shot or continuous mode, but it is not explicitly changed in the code, so it is likely also the default setting. In addition, to measure the voltage at A1 would I need to measure that relative to ground?

  • 0 in reply to Rohit Mokkarala
    TI__Mastermind 22950 points

    Hi Rohit,

    Thanks for the additional details. It sounds more like a grounding issue to me. The analog input should be measured with respect to GND but if the ADC has a different ground potential, that I would think that can cause some issues. For a sanity check, can you remove the 6F capacitor and connect a clean power supply and use a common ground? Does this help?

    Regards,
    Aaron Estrada