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ADS1158: ADS1158

Nir Dannenberg
Prodigy 20 points
Part Number: ADS1158

Hello,

The ADS1158 application in our circuit is to read voltages at the range of 0-5V

We are using external voltage reference of 4.95V instead of the example given 4.096V.

We are experiencing readings with increasing offset, where lowest offset at 0V and highest at 5V .

 
ADC In (V) V reading [HEX] V calculated per Vref DVM [V] V calculated per Vref ADC reading [V] Difference between Vpot and Vcalc per Vref DVM [V] 
0 13 0.003 0.003 0.003
1.014 1915 1.035 1.029 0.021
2.018 31d5 2.057 2.044 0.039
3.007 4a30 3.062 3.043 0.055
4.011 62e3 4.082 4.056 0.071
4.501 6ef5 4.580 4.551 0.079
4.85 774b 4.924 4.893 0.074
Vref DVM measured [V] 4.953
Vref ADC reading [HEX] 3b10

We played with reference voltages to see it solves this issue.

When reading the Code associated with Gain (V/V) (bottom of p. 24), I would expect that the code gain would change respectively.

However, I always read the same value.

1. Please explain why CODE does not change.

2. Please suggest why we experience non-constant readings offsets

Thanks,

Nir  

 

  • 0
    TI__Guru 52576 points

    HI Nir,

    If you get an approximately linear increase in input signal error from 0 to full-scale, this indicates a gain error. So you would need to calibrate this error out.

    As the datasheet says, the in order to measure the GAIN value, the ADC connects the external VREF voltage to the input and reference. Since the ADC output code is the input voltage (VIN) divided by the reference voltage (VREF), you are effectively always reading the value of VREF / VREF = 1 when checking the GAIN value. However, the actual value you read is not exactly 1x, and this should represent the ADC gain error. You would then scale all of your ADC output by this value, after first subtracting the measured offset.

    As the datasheet text also says, this does not compensate the VREF error, so this would need to be calibrated separately (or in lieu of just calibrating the ADC gain error you would calibrate the whole system error). If you have external amplifiers they can also contribute gain error.

    -Bryan