I'm having a problem with inaccurate reading from PCF8591P.
I attached self made Li-ion UPS to RPI and wanted to measure battery voltage, but it always differs.
PCF module is connected properly, Vin and Vref set to stablized 5.01 volts that doesn't even change a little.
When I connect a separate battery to AIN0 and GND (common ground for PI Zero and battery), then PCF module shows proper voltage, but when I want to check voltage of battery that acts as UPS (in fact I'm using it now when typing it), voltage is always different. My two multimeters shows 3.59 but data output converted from number received from PCF is, let's say 3.52. This is not even some kind of rounding as PCF is 8-bit module, so every step gives 0.020V difference but in my project, it reaches even 0.070V...
Did anyone of you had similar problems or is it some PCF modules limitation ? I use a simple chip, not any "magic board for 10 USD". Only two schottky diodes used and Vin<>GND 100nF capacitor.