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ADS1220: offset calibration

Amy Luo
Expert 6895 points

Part Number: ADS1220

Hi team,

I would like to confirm that when performing offset calibration on ADS1220 (as stated in the manual in subsection 8.3.12), does it require specific settings for other registers, for example, PGA must be enabled. The reason why I asked is that the two inputs of PGA are short to midvoltage supply ((avdd+avss)/2). Is the offset voltage of PGA calibrated？

  • 0
    TI__Guru** 113765 points

    Hi Amy,

    The short answer is yes, you should have the ADS1220 configured in the way you intend to operate the device. 

    Note Figure 38 in the datasheet where you can see the point in the input circuitry where the short is applied.  The PGA is a differential input with INA configuration, so the PGA will be included in the measurement if enabled.  If the PGA is bypassed, the setting for the desired gain(1, 2, or 4) should also be included when making the offset measurement.

    The PGA gain settings and the PGA enable setting will directly affect the outcome.  Also the data rate will affect the level of noise.  The reference settings will also determine the value of 1 output code (LSB).  The burnout current and IDACs will not determine the outcome directly.  If you are establishing a reference voltage using the IDAC then this will affect the outcome based on the reference voltage value established.

    So the key elements are PGA settings, reference settings and data rate (including Normal or Turbo mode settings).

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Expert 6895 points

    Hi Bob B,

    Your answer is very detailed, which is helpful for me to understand the use of ADS1220. Thank you very much for your reply.

    My question may not be accurate. I will describe the question as follows. Could you share your answer again:

    In the case of bypass PGA, can we also calibrate ADS1220 ? If PGA is bypassed, whose offset is calibrated?

    Looking forward to your reply!

  • 0 in reply to Amy Luo
    TI__Guru** 113765 points

    Hi Amy,

    The offset can be compensated with either PGA enabled or disabled.  The ADS1220 has gain options of 1, 2 and 4 with PGA bypassed.  So even though the PGA may be disabled, there will still be some potential offset characteristic in the pre-charge buffer and switched-cap gain circuit. 

    What is being determined with the short at the ADC inputs is the output code(s) that should be 0 for a 0V input, but is something else (either positive or negative). So you set up the device in the way it would be configured in normal use and then determine the code to be subtracted.  For example, let's say the code returned is 32 when it should be 0.  If I subtract 32 from 32 I would get zero.  If I apply another voltage to the input, I know that I will have a 32 code offset.  If the code value returned is 5395, then I would need to subtract 32 (5395 - 32 = 5363) and convert the remainder (5363) to the specific voltage for the reference being used to determine the conversion result.

    Best regards,

    Bob B 

  • 0 in reply to Bob Benjamin
    TI__Expert 6895 points

    Your support is outstanding. I really appreciate all your help.