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ADS1015-Q1: total error calculation

Part Number: ADS1015-Q1
Other Parts Discussed in Thread: ADS1018

Hi team,

I plans to use ADS1015-Q1 for voltage monitoring purpose in single end configuration.

The rail of monitoring is 3.3V rail with resister divider and supply voltage for ADS1015-Q1 is also 3.3V.

1. In single ended measurement, only +FS can be used, correct?

2. Could you advise how to calculate the total error (voltage tolerance of the target voltage rail)?

Any calculator or excel spread sheet?

regards,

  • Shinji-san,


    Thanks for your questions.

    1. For the ADS1015-Q1 in a single ended measurement, the ADC input is still technically differential. The AINP input is the analog input selected and the AINN input is ground. Therefore, your effective range is +FS. This is effectively an 11-bit ADC.

    2. For the total error, we often use the Total Unadjusted Error (TUE). This is calculated as the root-sum-of-squares of the errors of the device. It can be calculated from either the typical errors or the max errors in the electrical characteristics.

    You can read about the TUE in this document:

    www.ti.com/.../slaa587.pdf

    I have a similar write up for the ADS1018, here:

    e2e.ti.com/.../688599

    However, I've adapted these calculations for the ADS1015-Q1 below.

    Looking at the errors in the electrical characteristics for the ADS1015-Q1, the offset value of 0.5 LSB is still good (even if it is probably smaller than that). Also, the offset drift is 0.005 LSB/°C as a typical value. If you wanted a maximum value I would have taken the typical value and multiplied by 3x as a guess for the maximum. This would be 0.015 LSB/°C. If your temperature drifts from 25°C to 125°C then the drift would be about 1.5LSB.

    The INL on this device is very small, and is low sub-LSB for a value. Using 0.5 LSB is also ok but it is also likely much smaller than that. Regardless, it is also a small contribution to the TUE.

    The remaining gain error and gain error drift have the largest contribution to the TUE, as it is in your analysis. As you have, I would continue to use the max numbers as listed for the ±2.048V range. However, even if the input range setting is ±6.144V, the largest input signal that you can measure with the device is limited between GND and VDD. If the supply is +5V, the usable range of the device is ±5V. That would mean that the max gain error of ±0.25% is limited to ±4.16 Bits. This would be the number you use in your root sum of squares calculation.

    For the gain error drift, the 40ppm/°C number becomes 0.4% with a change of 100°C (assuming the worst case change from 25°C to 125°C). Again, with a 5V supply, the max gain error drift is ±0.4% translates to a gain error of ±6.67 bits.

    After that you can calculate the TUE from:

    TUE = SQRT ((0.5+1.5)^2 + (0.5)^2 + (4.16+6.67)^2) = 11 bits (this would be about 11mV in the ±2.048V FSR)

    This is the root-sum-of squares of the maximum errors. The gain error and gain error drift have the largest contribution for the TUE. The offset and INL have almost no contribution to this error. You can adjust this if you are limiting the temperature range of the ADC.  



    Joseph Wu