Since I am designing a meter (based on ATmega8) to measure the RMS current (50Hz mains power), I had a look at the application note “Atmel AVR465: Single-Phase Power/Energy Meter”.
In order to double the accuracy, I added a diode bridge between the transformer secondary and the sensor resistor (RS, Figure2-4 page 10).
In this configuration, the 4-diode bridge acts as an ideal full wave rectifier; this could be verified on a scopemeter.
Also increasing the reading scale could be done by simply decreasing the value of RS (for example, by adding another resistor in parallel).
I think if the current in the added resistor is made to be a few mA only at full scale, an MCU I/O pin could be used as a switch for it (as low-state output if on or tri-state input if off). This method will not need an external opamp and an IC for the electronic switches. But the MCU pin (used as a switch here) will be driven by the same ‘linear’ voltage of the ADC pin when it floats (configured as input with no pull-up). So I have to check what could be the drawback in this simple case. For example, I will monitor the increase of the current (Icc) drawn by the MCU (in my case, ATmega8 running at 8MHz by the internal RC oscillator) at various input levels and find out if it is acceptable or not (I will likely use 3 pins as switches to get 4 scales).