I have an external battery which is permanently connected to my device. I have a basic A/D circuit to read the voltage of the battery (current limiting resistor with some polarity and over-voltage protection diodes), which connects to a spare A/D input on my MCU. At the moment, this circuit is permanently connected to the battery, whereas the MCU is only powered on when the device is switched on. As such, the input on the MCU A/D is much greater than the MCU VCC (which is effectively zero), and hence in violation of the datasheet maximum specification. Similarly, it'd be great if the MCU A/D input was high/infinite impedance in powered down state, but it's safe to assume this isn't the case.
What's the most logical approach to this scenario? Ideally I'd be able to 'turn on' the A/D circuit when the MCU is powered, and turn it off again when it's powered down.