I'm reading the app note again and the part about alternating voltage bugs me. It says that "a 115 VAC 60 cycle sine wave will traverse from 0 to 2V in 32ms", what about 12VDC? Wouldn't that be on instantly so to speak? The car alternator may work with AC but it distributes DC to the car electrical system.
The other thing is the clamping diodes, they shut off over VDD and below VSS. That's fine to protect the PIC but that would interfere with my application. The car sensor works on 12V, I can't just stop measuring at 5V, I need "see" the remaining 7 volts or else I will get only a partial reading?
Let's use the fuel gauge as an example, the fuel tank would read from empty up to 40% full or so and then the meter would top out, no?
The only thing I can think of it to divide the car voltage by 4 and do the same thing with the signal voltage. That way I won't be comparing apples and oranges and I will be able to use the PIC ADC properly. My car battery is about 12.? charged and 14.? with the alternator charging. Let's use 16V as a maximum in case of a surge, divide by 4, that brings it down to 4V, well within the 5V limit of the PIC. I can still place a safety diode on top of that to protect at 5V too.
EDIT: Oh, and that spare 1V translates into another 4V buffer, that means it would be safe up to an absolute maximum 20VDC.
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