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ADS1248 - Offset

Kamal Najmi
Expert 3295 points
Other Parts Discussed in Thread: ADS1248, REF5025

we have a problem with a board from one of our customers that uses the ADS1248 ADC.

Our task is to make a redesign of the board to improve the measurement performance.

The current board is well designed and uses REF5025 reference sources.

 

When measuring in single ended mode and 5SPS we have a noise of max. 25 ADC counts (about 7,5 µV).

This would be ok for the application, but the measured value has a voltage dependent error.

When doubling the input voltage the error is  doubling too.

 

For example:

               

Input Voltage

Calculated

Measured

Error

203,46 mV

682.698

682.669

-29

806,91 mV

2.707.541

2.707.338

-203

1445,40 mV

4.849.958

4.849.453

-505

               

(The measured value is an average of 50)

 

When using faster sample rates this problem becomes worse too.

Also the self-calibration function of the ADS1248 didn’t solved the problem.

 

Can you explain this problem?

 

Thank you for your help.

  • 0
    TI__Guru* 92715 points

    Kamal,


    Looking at the numbers in your list, the "offset" varies with the input signal. This means that this isn't an offset but rather a gain error.

    If you take the largest reading and take the error and divide it by the measured signal (-505/4848453), you'll find that the error is about 0.01% which is within the specification of the gain error maximum of (+/-0.02%).

    Now to improve this there may be few possibilities. First, there may be large resistors in series with the analog inputs and the reference inputs, this added resistance creates a voltage divider with the apparent input impedance of either input, which can cause additional gain error. The customer can calculate what error they can expect by comparing the input impedance with the series impedance.

    Second, if they are having further errors, they can go into the Full Scale Calibration Coefficient register directly and alter the values to remove the error.

    Let me know if this helps. If you have further questions feel free to post back to the forum,


    Joseph Wu