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ADS124S08: When should SFOCAL be performed? On every power on, or once as a "factory calibration" with the offsets stored off in NVM?

Part Number: ADS124S08

I'm working on a design using 6 ADS124S08 devices measuring 32 thermocouple inputs. I've found I can get very good performance simply using the SFOCAL command prior to taking readings and don't need the SYOCAL or SYGCAL commands to get the performance I'm looking for. 

This has introduced a question: Since SFOCAL has such a small amount of overhead, I theoretically could run it on every power on. At my sample rate, it can complete the calibration process for all 6 ADCs in less than a second. However, I'm not sure that this is the best practice. 

Is there any documented "best" practice for calibration like this? Should I perform it once by manual command and store the offsets in memory to be accessed on power-on, or should I re-run the SFOCAL command every time the parts are powered on? 

  • Hi Taylor,

    Welcome to the E2E forum!  Unfortunately there is not a 'one size fits all' response.  There are some that never issue an SFOCAL as their system doesn't require it.  There are others that only issue at powerup while others will issue periodically either based on time or temperature to remove any offset drift.  So there are many possible options.

    There are a couple of components that affect the offset.  One is analog as it relates to input and PGA either enabled or disabled.  The other is digital as it relates to the modulator.  What the SFOCAL does is place an internal short to the ADC expecting a 0 code result.  The difference is applied to the OFC registers so that the result transmitted out of the device subtracts the offset.

    Before issuing the SFOCAL you should set the gain and data rate you intend to use.  I would recommend do this at device powerup as opposed to storing the register settings.  Storing the settings will work, but will not take into account temperature and any shift of offset over time.

    Best regards,

    Bob B

  • Bob, thanks for your reply. I forgot to mention, but I am setting data rate and gain prior to running the SFOCAL command, good to hear that's a good order. 

    Your description of the potential benefit of running SFOCAL on powerup makes sense and seems like a good path for me to go down. It also has the benefit of simplifying firmware!