I've got a circuit where I want to measure the voltage of the battery
supplying power. Since this is much higher than the VCC (I'm using a
regulator to provide power), I need to use a voltage divider resistor
network to drop the voltage to the correct input range. However this
resistor network draws power even when in sleep mode - I'd like to reduce
this as much as possible, since I don't want to drain my battery when it is
switched off, as it may be for long periods (hours, days).

First of all I can use high value resistors - however according to
datasheets the max input impedance is 10k, which would suggest that is the
maximum value of the lower resistance. Given I only need to measure every
second or so though and the voltage isn't varying rapidly, can I use much
higher value resistors and rely on not needing to charge the input capacitor
up very much each time, and so ignore the datasheet?

Secondly, are there any clever techniques I can use with FETs or other input
pins to cut of the current flow through the divider? At present I have the
ground end connected to an input pin and set that to a low output when
running, but to a high impedance input when going in to sleep mode - that
helps a bit, but I still have the current flow through the upper resistor
into my analogue input pin due to the clamping diode which prevents the
voltage there rising much above VCC. Got a couple of ideas using FETs, but
interested in anything anybody has done.

p.s. my apologies, this should in a way be OT, as it's actually being
implemented on an AVR at present, but given I've done similar things on a
PIC (in fact my development circuit was originally designed around one), and
the input requirements for the ADC appear almost identical to that on a PIC,
it is at least pretty relevant to that, and PIC based solutions others have
used will be extremely relevant!
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