Hi, johnuk.
According to the apps engr of this part, AD8138 is a higher-performance version of AD8132 in terms of noise and distortion. Thus AD8138 is as good as AD8132 with regards to these specs and so you may refer to the datasheet of AD8132 for the differential gain and differential phase errors.
Few questions:
1. Are you using composite video? These errors are only important in a composite video system.
2. Do you have a required differential gain and differential phase errors?
Regards,
Anna