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ADS1226: Multi channels and self calibration

Pierre Lamberbourg
Prodigy 40 points
Part Number: ADS1226

Hello,

It is not very clear for us when the self-calibration is needed.

We are using now the ADS1226 in high-resolution mode with the 2 channels active and performing a self-calibration each time we read a channel.

This gives use a maximum period at about 1/800ms (793ms) for reading 2 channels.

Today we need to have a higher sample rate and still need the precision. So I have 2 questions:

- Is the calibration performed valid for both channels?

- With a temperature that may vary within almost the whole operating range, how often do we need to perform the calibration?

Thanks a lot.

Pierre

  • 0
    TI__Guru** 113765 points

    Hi Pierre,

    Welcome to the E2E forum! This is a good question, but difficult to answer with respect to how often you should run the calibration.  The calibration itself is valid for both channels. 

    There are two key components to the calibration.  One is offset error and the other is gain error.  Both of these errors are subject to drift.  Gain error will also be subject to the accuracy and drift of the reference source.  So it is very difficult to say how often the calibration should be issued relative to the specific level of performance you require.  This means you will have to test your system. 

    If you have the ability to monitor temperature change on your system, you may be able to determine a valid range where your accuracy is not degraded.  For example, if your temperature changes by 20 deg C, you may need to issue a calibration.  Obviously you would need to keep track at which temperature points you calibrate.

    I would suggest to start by looking at the graphs on page 8 of the ADS1226 datasheet and in particular Figures 9 and 10.  This would be the typical response over the operating range of the device.  From here you can calculate the effects from the normalized value of room temp where a calibration is issued.  From Figure 9 you can see that offset drift is much more significant from -55 to 25 deg C than it is from 25 to 125 deg C.  The effect of gain error will also depend on the input voltage relative to the full-scale range.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 40 points

    Hi!

    Thank you very much for the fast answer. Much appreciated.

    Best regards,

    Pierre