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ADS1220: offset and gain drift specs

JAP
Intellectual 2410 points
Part Number: ADS1220

Dear Experts(Bob),

Few questions regarding ADS1220 offset and gain drift specs.

Input offset voltage:    Is ±4uV for single ended input, meaning that offset is ±8uV for differential input?

Offset drift:             Is it so that every single input can drift 0.25uV/C? If there is temp change of 10C, this means that single ended channel drifts 2.5uV and differential channel drifts 5uV?

Offset match:            Typical value is ±20uV. How can it be that high if typical offset value for single channel is ±4uV?

Reference voltage long term drift:        Can you expect long term drift to be linear? If it is 110ppm after 1000 hours, is it 220ppm after 2000 hours?

Thanks!

Rgds,

Juha

  • 0
    TI__Guru** 113765 points

    Hi Juha,

    I will try to explain the information in the Electrical Characteristics Table on page 7 of the ADS1220 datasheet for offset.  For the offset, there are offset numbers for PGA disabled and PGA enabled (2 sets of data for the PGA enabled where there is a min/max spec for a gain of 1 and a typical spec for gains 2-128) and these numbers are stating differential inputs.  The ADS1220 will always measure differentially even in the single-ended case where the PGA is disabled (and the only difference is full-scale range is reduced to 1/2 or only positive full-scale).  The typical offset is +/- 4uV for all cases, however there can be as much as +/30 uV (at room temperature) difference in offset.

    For the offset drift, there is once again 3 lines in the specification.  One is the PGA disabled case, and there are 2 lines for the PGA enabled case.  With PGA enabled one specification shows up to 85 deg C and the other for the full operating temperature range of the ADS1220.  Note that the typical values of offset drift is much less at 85 deg C and below.  If the temperature range is higher, then error will increase mostly due to transistor leakage.  Remember that all measurements are conducted differentially so there is no difference for a single-ended measurement.  The difference relates to PGA configuration and temperature for offset drift.

    The offset match specification compares the various input combinations on the ADS1220.  Based on averages there may be typically +/- 4uV offset seen on a device, however the spread of difference taking all combinations into account may see outliers beyond the typical.  The spread of the outliers is typically +/- 20uV.

    To further investigate the offset behavior I would suggest looking at the graphs for Figures 3 and 4 on page 10 of the ADS1220 datasheet.

    The reference drift shifts the most during the first 100 hours of the device operation.  After that there is some drifting up and down (it is not linear), but remains relatively flat.  I would not expect to see an overall shift even after the first 1000 hours to be more than the typical 110ppm from the initial reference voltage. 

    Best regards,

    Bob B