adc - Low current divider for battery voltage monitoring

I’m working on a low power sensor and would like to measure its battery voltage.

The battery voltage is too high my ADC input so I have been dividing it with a pair of 1M resistors and a 100n cap so that the ADC’s input capacitance doesn’t load it too much while sampling and affect the results.

The trouble is that the resistor divider drains a few uA all the time and this is far from ideal. Is there a common solution with a low BOM cost to this problem?

I thought of switching an N channel mosfet to the base of the divider but it would allow the ADC input to float too high. Putting a P channel on the +ve rail would be difficult to drive.

Do special ADCs exist that can measure beyond the +ve supply rail?

Any suggestions welcome.

Answer

Use a simple circuit like this. The P-Channel MOSFET can be some other low current device - The one shown is just the one that showed up in the schematic tool.

^{simulate this circuit – Schematic created using CircuitLab}

The idea is to connect the GP_OUT signal to your MCU and have the software drive that high at the time an A/D measurement is to be made. I show using 100K resistors for the divider but these may need to be scaled down depending on the input impedance of the A/D input pin. It's been my experience that 1Meg resistors used for this purpose lead to too much measurement error.

Some delay from setting the GP_OUT high will be needed till the A/D conversion is commanded to allow for settling of the A/D input.

Note that when the NPN transistor is off the only load on the battery is leakage current.