# Maximum voltage divider resistances for sensing LiPo battery voltage over 10bit ADC: is MOhm range working?

I've designed a PCB that should contain as few components as possible but it should have a simple way for measuring the battery voltage using an ADC channel and a simple voltage divider. My current design with 100k-220k division:

Consumes around 12µA, which is still a bit much for my project constraints.

I'd like to use resistances with 10x that value so around 1MOhm-2MOhm. The readings don't have to be accurate, I just want to have a rough estimate of the LiPo voltage to know when it is empty. I guess if there's too little current drawn over the voltage divider, the noise will be a problem for the ADC.

Is there a best practice for my application or some reference on estimating the noise caused by such high resistances?

EDIT: I am using an ATMEGA1284p AVR MCU, which has an input impedance of 100MOhm

• This 100% depends on the input impedance of your ADC (including it's resistive and capacitive load), together with the acceptable error. I'm guessing you're basically reading this once a minute, so it feels desirable to thing about noise, and directly add an RC anti-aliasing filter to this. Commented Mar 14, 2021 at 9:40
• @MarcusMüller I've edited my question with more information, I am using an ATMEGA1284p, the input impedance of the ADC should be 100MOhm, I am currently searching for ressources on RC anti-aliasing filters, can you please provide one? Found this here: ti.com/lit/an/sbaa282a/… Commented Mar 14, 2021 at 9:54
• first hint: don't use secondary sources if the primary sources are available: the ATMEGA1285P has a datasheet with an ADC section, certainly! Commented Mar 14, 2021 at 9:57
• So, look at the figure on page 251 of the current datasheet from microchip's website. You'll see that you need to charge a ca 14 pF sample-and-hold capacitor through your resistors, plus an internal 100 kΩ resistor. (the 100 MΩ is more about what you leak to ground or from VDD if you connected just a DC voltage that literally never changes). You can select the conversion time within bounds – so, pick a conversion time that's long enough, and you can use the biggest resistors in front of the ADC input that still allow charging of the capacitor within a conversion cycle! Commented Mar 14, 2021 at 10:10
• Thus, the choice of resistors here depends on the conversion speed you want to do. You'll really want to read section 23.7, ADC Noise Canceller; I've got this feeling "going to sleep and waking up when a loooong conversion is complete" is very much what you want. Commented Mar 14, 2021 at 10:11