Correct formula for BATT monitoring on expansion board

Hi
What's the correct formula for Battery monitoring with a LoPy on the pycom expanysion board.
According to the datasheet it's a 1:2 voltage divider on the expansion board. And the ADC is 12bit (04095) and somewhere i was reading the the reference voltage is 1.4 voltso the formula i came up is:
adcVoltage = adcReading * referenceVoltage / adcResolution
battVoltage = adcVoltage * voltageDividerFactormy code is:
from machine import ADC class LoPy(object): def __init__(self): self.adc = ADC() self.batt = self.adc.channel(attn=1, pin='P16') def get_batt(self): adcVoltage = self.batt.value() voltage = adcVoltage*3*1400/4095/1000 # print("Battery V = %.3f" % voltage ) return voltage
but the result is too high, while i measure 3.31 Volt with a fluke, the reading is 3.91 from the formula above
what could be wrong, my formula, the adc settings?


Ho do i calculate the range from dB attenuation?
So far i have the following Table:
attn = 0 = ADC.ATTN_0DB = 0 db gain = range 01V
attn = 1 = ADC.ATTN_2_5DB = 2.5 dB = range 01.334V
attn = 2 = ADC.ATTN_6DB = 6 dB = range 01.995V
attn = 3 = ADC.ATTN_11DB = 11 dB = range 0  3.548VThe Voltage divider consist of a 56k and 116k resistor witch gives a factor very close to 3
so i changed my formula to: voltage = adcValue * 3 * 1.334 / 4095
and it's pretty accurate nowmany thanks

@roadfox The basic ADC range is 1V. You can set an attenuation factor of 2.5 dB, 6 dB or 11 dB (https://docs.pycom.io/chapter/firmwareapi/pycom/machine/ADC.html). 11 dB results in a 3.548V range. The voltage divider on the expansion board consist of a 56k/100k resistor pair, resulting in an attenuation factor of the resistors of 2.79, resulting in a range of 0..2.79V. So you have to use an attenuation factor of the ADC.
Update: I see that you use ATTN=1, which is 2.5 dB, resulting in a ADC range of 0..1.334 V. But the ADC on ESP32 is anyhow not very precise, and the impedances are not known. So you have to determine your own calibration factor.
Update 2: Some discussion about that is here:https://www.esp32.com/viewtopic.php?t=1045
A while ago I made similar measurements with a Lopy. The table below shows results for the 1 V range (attn=0).Input ADC 70 0 100 127 200 550 300 974 500 1820 700 2650 900 3510 1000 3933 1044 4095
You see that it does not match the 01 rails. The same for attn=1. For attn=3 it is also nonlinear above ADC readings of about 2500.