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ADS1015: ADS1015 non-linear at upper end of range

Craig Easdale
Expert 2170 points
Part Number: ADS1015

Hello, I am using an ADS1015 to do some basic voltage measurements on two ranges, 0-5V and 0-12V. The ADC VCC is 3.3V, and I am using resistor dividers to adjust both of those inputs into the correct range. The ADC max input voltages are thus kept below VCC at all times. My PGA FSR is set to +/- 4.096V to allow for a full 0-3.3V input to be measured. The same rate is 16 SPS.

I am seeing good linearity at the lower end of the scale, but as I approach VCC on the input, the ADC measurement deviates and becomes non-linear. See the below chart of the voltage input applied (before the divider on the 5V channel), and the ADC output (scaled at the same factor as the resistor divider to represent the input value).

Why am I seeing this non-linearity, and what can be done to avoid it?

  • 0
    TI__Expert 5640 points

    Hello,

    Could you send me a basic schematic of the input structure you are using?

    As you have described it I wouldn't expect this nonlinearity, so I would like to verify this isn't being caused by some of the external circuitry. In addition to the data you took above, could you measure the voltage at VCC and the device's input, as close to the device pins as practical?

    Thanks,

    Levi DeVries

  • 0 in reply to Levi DV
    Expert 2170 points

    Hi Levi, thanks for the speedy response.

    VCC near the device is pretty spot on 3.3V. Schematic below.

  • 0 in reply to Craig Easdale
    TI__Expert 5640 points

    Thanks for checking that for me. Looking up the datasheet for the zener diode it looks like the diode will begin conducting before it reaches it's zener voltage: 

    If you try taking off the zener diode (or using one with a sharper breakdown characteristic) I think the issue will be resolved. You could also measure the voltage on the device pins directly to verify whether the diode is beginning to regulate the voltage down.

    Let me know if you are still having issues after this,

    Levi DeVries