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ADS5407 digital output format

Other Parts Discussed in Thread: ADS5407

Hi, Richard:

One more question below.

Currently our ADS5407 is working in default setting, that the Auto-Correction is disabled, with which, the Offset Error range, according to the data sheet on page 7, is between -20mV ~ +20mV.

My question is, for a same channel (there are two channels per package), in  one running ( after power on and before power off), its Offset is fixed, or is variable dependent on other factors?

Based on our testing, in which we tried different input voltage values ( not exceed range of 1.9V+0.25, 1.9V-0.25V), it seems the Offset Error is not a fixed value ( We compared two values, one is input differential analog voltage measured by multi-meter, another is ADC's digital output). 

I attached a pdf file that is a curve from actual measuring.

Thanks.

Jason8154.Curv-400MHZ _ Curve-TI.pdf

  • Hi,

    In offset binary format the output code goes from 0000 0000 0000 up to 1111 1111 1111 as the input voltage goes from its minimum up to maximum differential voltage.   With default full scale input voltage at 1V peak to peak differential and the signal being symmetrical and differntial about the VCM voltage - that means that the voltage at the input-P pin swings from VCM-0.25V to VCM+0.25V, and the voltage at the input-N pin swings with that range as well. 

    So if the *differential* input swing is 1V peak to peak, then the swing on each side of the input is 0.5V peak to peak, and the minimum input is when Input-P is at VCM--0.25V and Input-N is at VCM+0.25V.   Tha maximum input is when Input-P is at VCM+0.25V and Input-N is at VCM-0.25V.

    So if VCM = 1.9V, then the sample code 0000 0000 0000 is when Input-P = 1.65V and Input-N is at 2.15V (when the P side of the differential is 0.5V below the N side) .  The sample code 1111 1111 1111 is when  Input-P is at 2.15V and Input-N is at 1.65V ( when the P side of the differential input is 0.5V above the N side)   This is a differential swing of 1V peak to peak differential and spans the 12bit output codes from 0000 0000 0000 up to 1111 1111 1111.

    If the differential input is at 0V (like if the Input-P and Input-N were shorted together) then the output code would be about mid-scale or about 1000 0000 0000.

    Usually there is a diagram to illustrate this, but I don't see it this time. 

    If the format is 2's complement instead of offset binary then the output codes are the same except the bit D11 is inverted.

  • Hi, Richard:

    One more question below.

    Currently our ADS5407 is working in default setting, that the Auto-Correction is disabled, with which, the Offset Error range, according to the data sheet on page 7, is between -20mV ~ +20mV.

    My question is, for a same channel (there are two channels per package), in  one running ( after power on and before power off), its Offset is fixed, or is variable dependent on other factors?

    Based on our testing, in which we tried different input voltage values ( not exceed range of 1.9V+0.25, 1.9V-0.25V), it seems the Offset Error is not a fixed value ( We compared two values, one is input differential analog voltage measured by multi-meter, another is ADC's digital output). 

    Thanks.

    Jason

     

  • Hi,

    If you try a number of constant voltage input values, then I think you will be measuring some things besides offset voltage.  There are also specifications for nonlinearity and gain error.  If you were to plot input voltage vs output code then that should draw a straight-line slope and linearity can be thought of as the deviation from that straight line.  So if you look at the output code for one voltage and look at the output code for a different voltage, then it might not be the offset that is different but a different point on the linearity curve plus the offset.  If you sweep input voltage from min to max and gather all the codes, then average of those codes should be mid-scale and the offset can be seen as the difference in that average from mid-scale.  But yes, offset can vary over time.  Offset *may* drift with drifts in temperature or supply voltage.  Or if your input signal is not full-range but for some reason dwells in a region above or below the linearity curve then that can look like offset in the short-term.

    Regards,

    Richard P.