Multiple voltage measurement
The problem is that we come to the point where the real design (with component cost optimization etc) starts.

I suggest using AC resonant (50 or 60 Hz) pre-filter with low Quality Factor (5-10). The rectifier must be made of 2 low leakage low current low frequency rectifier diodes:







simulate this circuit &ndash; Schematic created using CircuitLab

Values and P/N of all components except R3 R4 are arbitrary.

R3 R4 give you an idea about the required total resistance and voltage divider ratio. I suggest not to use filter capacitor on the output or use moderate filtering: the ADC measurements must be synchronized to Mains.

The DC power +5 V for low signal electronics has to be delivered from Mains.

You can use a common MCU to control ADC and analog switch and to send data by some standard digital line. The latest has to isolated from MCU by a proper digital isolator (consider using AD SOIC-8 digital isolators or TI similar one: ISO7221C or M, ISO7220C/M). Built-in ADC (into MCU) is a good and cost effective option. The true 12 bits ADC is enough to get about 0.1 V (related to input) resolution and about 0.2 V (again related to input) typical error.

The care must be taken about:


Leakage currents of D1 D2 over the whole intended temperature range (they must be much smaller than nominal current of R3 R4 divider: less than 1 uA, this may be difficult)
Voltage rating and AC performance of C1
Cost effective and compact design of L1
Precision and temperature coefficient of R3 R4
R2 can be either equal to R3+R4 for symmetric load or small - for double voltage rectifier (not recommended)


As for the switch - look for a suitable one for example on AD site. You need 5 V unipolar switch - they are good and relatively cheap.

The number of channels per one ADC input depends on conversion time (sampling frequency) of the ADC. I use typically 1 MS/s ADCs. Then you can sample 16 channels in 20-30 us.