The performance specifications listed in the data sheet are for a +/-250mV input signal. If this input signal range is exceeded the AD7403 will continue to operate but the specifications are not guaranteed. So to maintain accuracy it is recommended to keep the input signals into the specified performance range.
The maximum full scale positive input where the modulator will output all 1's is +320mV. The maximum full scale negative input where the modulator will output all 0's is -320mV.