LTC2496 producing a spike in voltage while reading

Hi,

We are using LTC2496 ADC which is connected to our microcontroller F280025PTSR through SPI . In LTC2496  CH0&1, CH2&3, CH4&5, CH6&7 and CH8&9 are configured  as differential channel, where as CH 10 ,CH 11 are  configured single ended channels. 

Channel 10 and 11 are working as expected and the ADC results are matching with the input voltage. 

Below is our voltage calculation  

if MSB=0

   Voltage=Raw data* LSB    i.e LSB=0.5 * Vref / 2^n

if MSB=1 

    data=Rawdata-2^n

    Voltage=data*LSB         i.e LSB=0.5 * Vref / 2^n

For testing purposing , the differential voltage for CH 0 & 1 has fixed input voltage of 0.8V, and when we are trying to read the ADC the raw data is either 0x268CD0 or 0xFFFFFF. We have observed that when we are reading data from ADC channel 0&1, the ADC is adding 0.204V to the our external fixed voltage of 0.8V and we read the data we are getting 1.04V after converting. 

Can you please let us know the reason why there is a spike in voltage when we are trying to read ADC and also why we are getting 0xFFFFFF0.

Regards,

Revathi