Well, I think I have fully understood the problem with the KC868-A16 board.
And it is a problem with the board hardware. With the current electrical circuit, this board cannot even theoretically measure input voltages in the range from 0 to 5V.
Let's look at a slightly simplified scheme of analog inputs 1, 2, 3, 4.
This is the actual scheme of my board and it matches the scheme in the description quite well.
And yes, inputs 1 and 2 of my board are current, and 3 and 4 are voltage.
As can be seen in the scheme, inputs 3 and 4 have a voltage divider - resistors R3 and R4. With these resistance values of these resistors, the coefficient of this divider is K=0.95. That is, if you apply a voltage of 2.0V to analog input 3 or 4, then the voltage of 2.0*0.95=1.9V will be supplied to the input of the operational amplifier U21, and when applying a voltage of 4.0V, we get 4.0*0.95=3.8V. Operational amplifier U21 is connected as a repeater, its gain K=1, which means the same voltage will appear at its output.
But assembly of two Schottky diodes BAT54S is connected to the output of the operational amplifier.
The Schottky diode has a forward voltage of about 0.4V. If the output of the operational amplifier is supplied with a voltage of 3.3+0.4=3.7V, the upper in the scheme Schottky diode will open and further increase in voltage will be impossible.
Thus, measuring the input voltage of the board's analog inputs with such element values is only possible for values no greater than 3.7V.
To fix this problem, you need to change the analog input divider circuit - replace the 510ohm resistor with a 5.1k resistor.
In this case, its division coefficient will be equal to K=0.66 and then when applying a voltage of 5.0V to the analog input, the voltage at the output of the operational amplifier will be 3.3V, that is, the Schottky diode will not open yet, the circuit will work correctly in the range up to the required 5V.
These resistors values are used on the board KC868-A6 that I also have and it measures input voltages in the range from 0 to 5V quite correctly.
But I am afraid that you have manufactured and sold many copies of the KC868-A16 board with incorrect analog input scheme. You should warn buyers about this. Or you can change the board description to indicate the input voltage range for analog inputs from 0 to 3.7V, where the board can work correctly.
And it is a problem with the board hardware. With the current electrical circuit, this board cannot even theoretically measure input voltages in the range from 0 to 5V.
Let's look at a slightly simplified scheme of analog inputs 1, 2, 3, 4.
This is the actual scheme of my board and it matches the scheme in the description quite well.
And yes, inputs 1 and 2 of my board are current, and 3 and 4 are voltage.
As can be seen in the scheme, inputs 3 and 4 have a voltage divider - resistors R3 and R4. With these resistance values of these resistors, the coefficient of this divider is K=0.95. That is, if you apply a voltage of 2.0V to analog input 3 or 4, then the voltage of 2.0*0.95=1.9V will be supplied to the input of the operational amplifier U21, and when applying a voltage of 4.0V, we get 4.0*0.95=3.8V. Operational amplifier U21 is connected as a repeater, its gain K=1, which means the same voltage will appear at its output.
But assembly of two Schottky diodes BAT54S is connected to the output of the operational amplifier.
The Schottky diode has a forward voltage of about 0.4V. If the output of the operational amplifier is supplied with a voltage of 3.3+0.4=3.7V, the upper in the scheme Schottky diode will open and further increase in voltage will be impossible.
Thus, measuring the input voltage of the board's analog inputs with such element values is only possible for values no greater than 3.7V.
To fix this problem, you need to change the analog input divider circuit - replace the 510ohm resistor with a 5.1k resistor.
In this case, its division coefficient will be equal to K=0.66 and then when applying a voltage of 5.0V to the analog input, the voltage at the output of the operational amplifier will be 3.3V, that is, the Schottky diode will not open yet, the circuit will work correctly in the range up to the required 5V.
These resistors values are used on the board KC868-A6 that I also have and it measures input voltages in the range from 0 to 5V quite correctly.
But I am afraid that you have manufactured and sold many copies of the KC868-A16 board with incorrect analog input scheme. You should warn buyers about this. Or you can change the board description to indicate the input voltage range for analog inputs from 0 to 3.7V, where the board can work correctly.