TMS320F280049: Why ADC gain error marked greater while using internal reference than that while using external reference?

Hi Arthur,

In internal reference, the gain error includes the inaccuracy of the on-chip bandgap voltage reference (over temperature, voltage, and process variation).

In external reference, the error in the voltage reference is not included (because it is off-chip and unknown).  You'd need to include any inaccuracy in your external source on top of the +/-5 LSBs. 

For instance, if you used REF3030 to supply the external reference voltage, buffered by OPA320, operating over the full range of -40C to 125C, then the gain error would be approximately:

• REF3030 0.2% initial accuracy = 8 LSBs
• REF3030 75ppm max temp drift * (125C - 25C) = 0.75% = 30 LSBs
• OPA320 Vos = 150uV = 0.2 LSBs
• ADC external gain error = 5LSBs
• Overall gain error = sqrt(8^2 + 30^2 + 0.2^2 + 5^2) = 31.5 LSBs

Which is still better than the internal reference gain error (but the external ICs have some cost and board area use, so some applications will use the internal reference).