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I designed an voltage divider and I have a problem (misunderstanding) when turning ON one of the transistors

For Q2, I realise that the GND is in the others drain pin transistors (Q4, Q5).

Other problem is the input voltage can be up to 48V, and i want to measure it with an microcontroller, so when all the transistors are off the voltage at the ADC pin will be 48V.

enter image description here

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If you make a chain of resistors, e.g: GND - 300R - 302R - 820R - 8.9K - EXBAT_IN then intermediate points will be at the same ratios as in your schematics. Then you can use FETs to connect AIN0 to these intermediate points and there will be no danger of exceeding ADC voltage max (since when all FETs are OFF nothing is connected to ADC).

Another option is to remove Q2 and wire R31 to ground permanently. Then re-calculate R32, R33 for parallel connection with R31. Not only you will protect AIN0 from overvoltage, but you will save one control line.

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Add a zener diode from AIN0 to ground, with its anode connected to ground. Select the zener voltage so that it is less than the maximum allowable voltage at AIN0. The zener will clamp the input voltage when none of your transistors is conducting and prevent damage to the microcontroller.

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I suppose "For Q2, I realise that the GND is in the others drain pin transistors (Q4, Q5)" means that more than one transistor is turned on so the equivalent lower resistance of the voltage divider is formed by their resistances in parallel?

To protect the microcontroller input from the overvoltage ("when all the transistors are off"), you can connect an ordinary diode between the microcontroller input and +VCC - the anode to the input and the cathode to +VCC. It will act as a diode limiter - when transistors turn off, it will turn on and fix the input voltage at +5.6 V.

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