I am unsure about the transfer characteristic of the AD976/AD976A shown in page
10 of the datasheet.
The analog input range is -10V-10V and reference voltage is 2.5V, the transfer
function should be that shown (table 1 in attachment).
The full-scale range (FSR) of the AD976A is equal to ±4 VREF (VREF=2.5V), so
the with a ± 10 V input, one LSB is 305 µV.
The AD976A transfer characteristics on page 10 can be shown as below (table 2