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ADS1261EVM: Using caibraiton commands

Part Number: ADS1261EVM
Other Parts Discussed in Thread: ADS1261, ADS1255

Hi,

I'm just getting started with the ADS1261EVM; I want to pick a channel and calibrate it; I've found the SYOCAL, SYGCAL and SFOCAL commands but what do they do exactly?

Do they calibrate all channels in one go? If not, how do I pick a single channel?

Other questions are:

Does SYOCAL short the PGA inputs or is it for a full system calibration, i.e. including my external circuit?

Assuming SYGCAL does a full system calibration, what gain is it set to?

What does SFOCAL do exactly?

Do any of these commands connect in Vcom? If not, how do I do that using the commands available?

Do I need to use these commands in conjunction with others?

Do you have any script examples I can use?

Any other insight would be greatly appreciated.

Many thanks,

Elton

  • Hi Elton,

    I wrote a fairly detailed description about self vs system calibration modes on this other E2E thread: https://e2e.ti.com/support/data-converters/f/73/t/707084. It was written for the ADS1255 but should still be very applicable to the ADS1261.

    Regarding your other questions...

    • Calibration does not mux through multiple channels. It only calibration the current channel, MUX, and PGA configuration. To calibrate multiple channels you'd need to re-configure the ADC, run calibration, and then store the calibration results (in your MCU) for each channel.

    • The gain used during gain calibration will be unchanged from what you've programmed in the device registers. So, if you set it to 16 V/V, the gain calibration will be performed with a gain of 16 V/V.

    • The only calibration command that changes the MUX settings is the self-offset calibration (SFOCAL). In this mode, the MUX internally shorts the inputs together to a mid-supply voltage to measure the ADC's offset.

    • I don't have a generic script that would apply to all applications, but in general you would configure a single channel as you would for a normal measurement, but then run two different calibrations:
      1. Short the inputs to the system and run the system offset calibration command.
      2. Input a FS signal to the system and run the system gain calibration command. NOTE: Sometimes it isn't always practical to provide a highly accurate 100% FS signal. The gain error will only improve if the error of the calibration source is LESS than the uncalibrated gain error of the ADC. Therefore, sometimes gain error calibration is skipped.
      3. If needed, the values of the offset and gain calibration registers are stored in MCU memory and recalled each time the ADC is configured to collect data from the current channel.

    I hope that helps!

  • Hi Chris,

    thanks for all the info - very useful. To be clearer however, I only want to calibrate ONE channel, the problem is that I can't see how to select ANY of the channels right now.

    I'm just using the Eval Module out of the box and only have the available commands, so I can't see how to select a particular channel, nor can I see how to set the gain.

    Many thanks,

    Elton

  • Hi Elton,

    From the EVM GUI you should be able to view all of the device registers and configure the settings...

    Click on the PGA register and you'll get some drop-down menus to configure the PGA gain.

    Then click on the INPMUX register and you can select the input pins to connect to the PGA inputs.

    I hope that helps!

  • Hi Chris,

    thanks for that, and apologies, I hadn't looked properly, so that was a bit dumb of me :)

    On another note, if you don't mind, I do all that and run the SYOCAL but I'm getting the FAULT LED (D1) coming on.

    For information:

    - I've got my external wires shorted during SYOCAL.

    - I see that bit 5 In the STATUS register is getting set, but in the document I have it says that bit is Reserved, Read Only and should always be 0.

    However, when I read the STATUS Reg in the Console, it says "PGA low alarm".

    - For what it's worth, bit 2 in the STATUS register is also set, which indicates there's more data to read.

    Can you help decipher this please?

    Many thanks.

  • Hi Elton,

    It's not a problem at all, and I appreciate knowing what parts of the GUI are not intuitive so that we can hopefully improve upon that in the future.

    Regarding the STATUS errors you're seeing, it sounds like you're shorting two of the inputs together, but not connecting them to anything else, is that correct?
    In that case, the differential voltage is 0V, but the common-mode voltage is left floating. If this voltage floats to either of the analog supply rails (due to some current leakage or the input bias current of the ADC) then the PGA outputs will also saturate as they try to drive to the supply.

    • Bit 5 in the STATUS byte or STATUS register is a read-only register, but it can go high to indicate that one of the PGA outputs is trying to drive to 0V. The flag is there to warn you that the conversion result may be inaccurate due to this non-linear effect.

    On the ADS1261EVM, once a STATUS error flag is detected, the FAULT LED is latched and will remain on until you clear it (either by pressing S3, or sending the "CLEAR" command)

    To calibrate the offset without triggering either of the PGA alarm flags, I would recommend shorting both ADC inputs to REFOUT or some other mid-supply voltage that is within the common-mode range of the PGA. For more details about this, I would recommend downloading the Excel ADS1261 Design Calculator (Rev. A) and experimenting with the "Common-Mode Range" tab to understand how the PGA behaves with different input voltages at different gains.

  • Hi Chris,

    that's exactly right yes, I've shorted the inputs but it didn't occur to me that the common mode voltage would/could be a problem, so thanks for the guidance.

    As a detail, can I double check what to set Vref to in the Data Analysis window? As I'm using the internal 2V5 I've left it at 2V5 as well, correct? 

    It's just a little bit confusing because I don't have an external voltage ref to do a full scale cal. with, hence can only do the offset cal.

    Thanks,

    Elton

  • Hi Elton,

    Unfortunately, there is no way to measure the absolute voltage of the internal 2.5V reference without some other precision (reference or input) voltage to compare to.  However, it's not uncommon for many ADC applications to only calibrate for offset, and leave the gain error un-calibrated. It's not always practical or cost effective to include an external precision reference voltage on the PCB or to calibrate every system by hand in a lab.

    For the GUI, I would just leave the voltage reference parameter set to 2.5V. Changing this voltage only has the effect of scaling the voltage (Y-axis) displayed in the time-domain plot.