I have similar problem with TMS320F2812.
We are getting error in result of ADC varying at varying voltage levels.
Same is observed on our customized board as well as TI development board.
Error is more at voltages below 200 mV and above 2.8 Volt in mid range that is from 0.2 Volt to 3.3 Volt the erro respone is quite linear whci may get solved by TI ADC calibration procedure.
I would be happy if somebody suggest solution for ADC error correction at lower and higher voltage band as mentioned above.
We using ADC at lowest frequency i.e. 2 MHz.
I had discussion with TI support staff also for the same.