isolate ADC & transformer in terms of supply?
If you are trying to make an isolated ADC measurement then I would consider this idea - it isolates the digital inteface rather than an analogue interface: -



On the left is the ADC and it would directly connect to the transformer (voltage, current measurement etc.) with suitable resistors. The device that is centre page is a data and power isolator for the ADC and looks internally like this: -



So, power to the ADC is ultimately derived from the microcontroller interface on the right and all data and power is isolated so that the ADC can make a direct connection to whatever needs to remain galvanically disconnected from the microcontroller.

I'm not saying that the AD7793 is the right choice for your ADC because that ultimately should be something that you decide upon however, there is a related family of ADCs from ADI (all with the same interface) and it's quite likely that one of them will suit.