Hi DonWarr,
Apology for the late reply.
The AD7192 has internal calibration so, if the offset/gain of the ADC only is being calibrated, the internal calibration can be used. The system calibration is useful if you wish to calibrate the offset and gain of your system. For a system calibration, you can use external switches to switch in appropriate voltages. The FS voltage should be close to VRef/gain. The ADG888 has low on resistance. So, it is a good device to use.
Regards,
Johnny