Phase lag of an ISO124 isolation amplifier?
If you assume single pole 50kHz response (which is obviously not 100% accurate) the phase shift becomes simply: 

\$\phi = -\tan^{-1}(100/50000) \approx -0.11°\$

It's not a linear system- it modulates and demodulates to cross the barrier- so 'phase shift' is only going to be an approximation anyhow. You can see significant artifacts and aliasing as you approach the modulation frequency. 

There is doubtless some some small amount of 'dead time' in this kind of system (should be less than 1/fmod or 2usec nominally) so very small angles may not follow the approximation so well. But 0.11° is quite a bit more than that (about 30usec).