ADS124S08: Do you have any graph of Gain/Offset/Int vref temperature drift?

Hi Kazuya,

Generally it is recommended to make the measurements ratiometric whereby the bridge excitation is the same voltage as the reference voltage.  The ratiometric measurement will cancel drift and most noise within the excitation source.  The SFOCAL will remove the offset of the ADC and usually any gain error is very small as the ADC range relative to the range of the bridge used for the weight scale is small.

Q1. Do you have any graph (X axis=temp, Y axis=drift value) of Gain/Offset/Internal voltage temperature drift? [Bob] The graphs for the typical drift appear in the ADS124S08 datasheet in figures 19-21.

 Q2. Is the change tendency of these temperature drift linear or quadratic curve? Or is it difficult to say clearly because it is depend on production lot or other reasons? [Bob] The graphs show typical drift from characterization, but the actual drift will be device and lot variation specific to within the max specifications in the Electrical Characteristics table.  Generally as temperature increases the internal components will begin to leak which is more obvious in the offset and gain drift graphs.  The internal reference voltage is trimmed to stay within a region over temperature.

 Q3. Typically, are such graphs not included in ADC datasheet? [Bob] Most recent devices will include these characteristic graphs for precision ADCs.

Best regards,

Bob B

Hi Kazuya,

Here are my responses:

 Q1. I can see on Figure 19 that the offset voltage temperature drift increases when the temperature rises.

        Is this offset voltage absolute value because I think the offset value has positive side and negative side? [Bob] Yes the offset can be either positive or negative.  The curves show a typical response based on device characterization.  Initial offset can be minimized by issuing the SFOCAL which is the self-offset calibration.  This will be the offset to the level of noise.  Additional periodic SFOCAL can adjust for temperature variations.  Or the global chop feature can be used to remove the offset for every conversion.

 Q2, I can see on Figure 20 that the gain temperature drift goes to negative side only when the temperature rises.

        Is this tendency not changed by production/voltage variation? Of course I know the value is change by the variations. [Bob] Again, these graphs are typical graphs of the types of trends you would see with respect to the gain error.   As with the offset there can be variations and the total possible error should be determined from the Electrical Characteristics table.

 Q3. If I calculates total temperature drift value with offset error and gain error, how we should calculate the drift value with PGA gain is 128? [Bob] It is better to look at the trends from the graphs instead of looking strictly at the min/max values.  Look at the trends in the graph over the temperature region of operation at the desired gain and calculate the drift from the graphs.

        Is it that "(Differential input voltage + the maximum offset drift (200uV/degree C)) * the maximum Gain drift (4ppm/degree C)"? [Bob]  Again I would suggest looking at the trends to get a more realistic approach.  The offset can be dealt with by either SFOCAL or global chop.  The gain error will be most dominant the closer the result is to full-scale.  The total error is an RSS analysis using the same unit values as ppm or uV, so you may have to convert the values.

Best regards,

Bob B