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ADS1232: Help me find Offset voltage for the part "ADS123x"

Jagrutkumar Vaghela
Prodigy 50 points
Part Number: ADS1232

Hi,

How to find the offset error in voltage for the part "ADS123x",

i checked the datasheet and it says ,

So based on that i calculated that for gain=128 and Vref=2.5,

FSR = Vref / Gain

FSR = 2.5/128

FSR = 0.01953125 

Now 

(0.02*0.0001* 0.01953125) / 100 = 0.000000000390625 or "390pV"

where 0.02 = offset error at 128,

0.0001 is PPM.

My Queries are

1) As per datasheet , its "0.02 PPM of FS" , can i assume there is typo and it is FSR and not just FS?

2) my assumption is that the unit for the offset error is (V), is this correct?

3) is the offset error calculation done above is correct? or please suggest the right way to find it.

4) if the calculation is rightly done, can i assume the 390pV as the offset error of the ADC and the minimum sensing ability of the same?

  • +1
    TI__Mastermind 41010 points

    Hello Jagrutkumar,

    The offset voltage is independent of the actual reference voltage, and the offset specification is based on VREF=5V only.  Using Gain=128, the full scale input range with respect to the offset specification will be FSR=5/128=0.0391V.

    PPM is parts per million, so the correct scaling factor is 10^-6.  

    Offset Voltage Max +/-1ppm (G=128):

    +/-1*10^-6*0.0391 = +/-3.91*10^-8, or +/-39.1nV (nanoVolt)

    Offset Voltage Typical +/-0.02ppm (G=128):

    +/-0.02*10^-6*0.0391 = +/-7.81*10^-10, or +/-0.78nV (nanoVolt)

    These specifications are only valid after offset calibration (section 8.4.1 of the datasheet).  In addition, if offset recalibration is done over temperature, the maximum offset error will be minimized to the noise level.

    Answers to your specific questions:

    1) As per datasheet , its "0.02 PPM of FS" , can i assume there is typo and it is FSR and not just FS?  Yes, this is FSR.

    2) my assumption is that the unit for the offset error is (V), is this correct?  Yes, the units are Volts.

    3) is the offset error calculation done above is correct? or please suggest the right way to find it.  Not quite; use maximum Vref=5V for these calibrations.

    4) if the calculation is rightly done, can i assume the 390pV as the offset error of the ADC and the minimum sensing ability of the same?  The typical offset will be 2x higher, or 0.78nV.  However, this value is much smaller than the noise of the ADC, which will be the limiting factor in sensing ability.

    Regards,
    Keith Nicholas
    Precision ADC Applications