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ADS125H02: Calibration form the ADC

Part Number: ADS125H02

Hi,

I am using this ADC  to calculate the voltages that are in the range of  0.9V to 12V.I am able to perform the offset and gain calibartion on the ADS125H02.

Offset calibration procedure:

I have enabled the Internal short to VCOM in mode4 register and given the offset calibration command and the offset calibration registers are getting updated by ADC.

Gain calibration procedure:

I have taken the 5vref as an input and set the pga gain as  0.5 and issued the gain calibration command and the gain calibration registers are getting updated by ADC.

After that calibration sequence is done I am setting the pga gain as 0.125 and taken all the readings from the voltages that are in the range of 0.9V to 12V.

For 12V I am getting only the 10.6V. For 1.8V I am getting only 1.7V .For 0.9V I am getting the 0.83V.

Why the voltages are getting this differnce even after performing the calibrations?

Is there any solution for this problem??

I just want to know that is there any internal voltage that can be used for the calibration of the ADC.

 

  • Hi Ramesh,

    The full-scale calibration coefficient is specific to a gain setting. In other words, you must perform the calibration with the gain setting that you intend to use e.g. 0.125 V/V, otherwise the scaling will be off. You can also read back the value of the OFCAL and FSCAL registers to see if they are approximately correct i.e. they are not huge values that are skewing your results.

    Also, there is no guarantee that calibration is causing whatever issues you are seeing. There could be grounding issues, layout issues, bad connections to the sensor source, improper decoupling, drooping supplies - the list goes on. You can try playing around with the calibration for now, but if that doesn't work please send your register settings and a schematic.

    You can use the ADC's REFOUT pin as a 2.5V input to the ADC for gain calibration, although this will not calibrate the gain error of the internal VREF, only the ADC.

    -Bryan

  • Hi Bryan,

    I can't use the input voltage as the calibration input voltage.As I specified in the mentioned mail the voltages are in the range of 0.9v to 12v.Suppose if I use the ADC REFOUT pin 2.5V as input calibration voltage I need to apply the PGA gain setting as 1. But for my voltage range I need to use the PGA gain setting as  0.125 and 0.1875.

    So Bryan Please suggest a correct way in which I can do the calibration for the above query.

  • Hi Ramesh,

    In that case, you need to use a precision source that generates a full-scale voltage at the gain settings you intend to use. This could be a precision resistor divider (maybe a matched array?) with a low-noise buffer. Or you can use bench-top equipment as well, assuming you only need to perform the calibration procedure one time.

    -Bryan

  • Hi Bryan,

    We are using the unipolar supply (0V to 12V ) to the device and would like to measure input range from (0.9V to 12V).The voltage above 2.5V and below 9.5V should be correctly read with our HW software..So, do we need to perform calibration in this case or not?

    I am getting PGA Alarm (STATUS1) Register (address = 11h)  is getting the value 0x88.That means the bit7 and bit3 of that register (PGA Output Negative Low Alarm and PGA Input Negative Low Alarm) respectively are getting the alarm active.Why these alarms are getting active ??

    I have attached the schematic for your reference in this thread.

  • Hi Ramesh,

    It is really up to you to determine if calibration is needed in your system. You have to define an accuracy target and determine if your system meets this target. If the system does not, there could be lots of reasons why, many of which are unrelated to the ADC performance. You would then need to diagnose the circuit, identify the error source(s), and correct them. Again, this is not necessarily related to the ADC.

    The PGA monitor behavior is described in section 9.3.3.2 in the ADS125H02 datasheet. Since your full input signal range (0.9V to 12V) is outside of the PGA operating range for your supply range (AVDD = 12V, AVSS = 0V), this is why you are seeing errors. You can reference the ADS125H02 calculator tool to better understand the valid input ranges for your system configurations: https://www.ti.com/lit/zip/sbac234

    -Bryan