ADC Saturation Problem

[dpj]

Hi,
Working with Freescale 56F8037 controller with 12 bit ADC for motor control.
Input voltage range -10V to +10V
ADC High reference = 3.3
ADC Low reference = 0V

ADC values are getting saturated at +9V ( Instead of +10V )

I have computed ADC offset factor and applied but this non linear curve at the edge of range is bothering.

Any direction to solve the problem?

Regards

 

[alec_t]

Could the saturation be due to the analogue conditioning (gain, offset) being applied? Reduce the gain, adjust the offset?
If an op-amp is doing the conditioning have you allowed for its ouput not going fully rail-to-rail?

 

[dpj]

Op Amp used are ±15V range so, pre-conditioning of the signal is little remote possibility!!
Thanks for the insight!!

 

[alec_t]

Looking at your table, I see there's a generally steady increase in the error. That suggests to me the op-amp gain is too high. Have you allowed for the op-amp input resistance in calculating the input and feedback resistor values? The op-amp input resistance is effectively in parallel with the external input resistor so will increase the gain over the calculated one.