Hello
I am trying to work with AC excitation for the ADS1262 and for that I am following the document ``High-Resolution, Low-Drift, Precision Weigh-Scale Reference Design with AC Bridge Excitation´´. But I have a question about it, how often do we have to change the excitation polarity in order to properly remove the offset? What is the correct change time?
Thank you in advance.
Best regards,
José Huertas Pedroche
Hi José,
Referencing Equation 3 in the the Design Document for TIPD188, VOS2 is only removed from the measurement result if its amplitude does not change significantly between the measurements in Phase1 and Phase2. NOTE: There is a typo, equation 3 should evaluate to VIN + VOS1, as shown below:
So to answer your question, it depends on the source of the offset voltage that you are trying to remove (represented by VOS2). Typically, VOS2 will have a strong temperature dependence and so it will tend to be a slowly changing offset. With this in mind, you don't need to chop very fast to remove temperature related offsets.
To find the "optimal" chopping speed would probably take some testing at different data rates...
Keep in mind that there will probably be a trade-off between noise performance and accuracy. As you chop faster, you will remove more of the offset voltage but will degrade the noise performance by having to run the ADC at a faster data rate. And, by running the ADC at the slowest data rate you will have the best noise performance but may have a slightly larger offset (since a more slowly changing offset may still have some residue offset voltage left after the averaging).
