Hi - I'd like to measure the voltage of a ~55V DC power source with the ADC of a 3.3V MCU. Whenever the MCU is on, I would like to be measuring the voltage of this power source. When the MCU is off, all of its pins go high impedance. I don't want to hang a normal resistive divider to ground across the power source, as that'd be a constant load (even if that load was in the uA range). Instead, I'm thinking about putting a resistive divider on it, with a FET switched ground. The gate of the FET would be driven by the microcontroller with a 10K pull down to ground. I'm thinking I'd use a 2M (1%), a 105K (1%), and a BSS138 (RDSon =~ 1.5). Nothing special about the BSS138 - I just have a bunch of 'em hanging around. Looks like it'll let half a uA through when turned off. More than I'd like - but I can probably survive that.
Is this a bad idea? The ADC will be using a VREF of 3.0V. The MCU will shut off when the power source goes down to 36V or below. How do FETs handle low current situations like this? Will I see a large VDS? The BSS138 datasheet is more interested in higher current situations. Maybe there are FETs better suited for this?
Thanks!
-Michael