Measuring +12V with A/D in car environment

[giszo]

I would like to measure the +12V supply of a car with a microcontroller that takes 3.3V as its analog input at most. I know I have to use a voltage divider to shift the input voltage to the acceptable range.

The missing part is the filtering of the sensed voltage. What kind of filter should I use on the input to eliminate spikes generated by the alternator and other components of a car?

I have tried to google this question but it seems that everyone is trying to build a filter for a car power supply that is not the thing I am looking for.

 

[barry]

I would just throw a 10K and 10uF on there, buffer it with an opamp, and see how it looks. You might have trouble filtering narrow, high voltage spikes, but you can also do some averaging in software that will smooth things out.

 

[giszo]

you can also do some averaging in software that will smooth things out.

That's true, however I am afraid that the input of the ADC will not like those high voltage spikes.

 

[barry]

That's true, however I am afraid that the input of the ADC will not like those high voltage spikes.

You can add a zener or transorb.

 

[Easyrider83]

Input resistance of analog pin is about gigaohms. Opamp doesn't needed. Just devider 1:8 - 1:10 and capacitor.
Lets say 10k/1k + 100n will be enought.

 

[mjasoj]

Designing filter for a very wide range of input (including spikes) might be a difficult task, you may better prefer some kind of protection circuitry for microcontroller.

May be done using a zener diode and a strong power resistor for high power dissipation

 

[giszo]

Input resistance of analog pin is about gigaohms. Opamp doesn't needed. Just devider 1:8 - 1:10 and capacitor.
Lets say 10k/1k + 100n will be enought.

The capacitor should go between the ADC input and ground? (in parallel with one of the resistors from the divider)

 

[barry]

Input resistance of analog pin is about gigaohms. Opamp doesn't needed. Just devider 1:8 - 1:10 and capacitor.
Lets say 10k/1k + 100n will be enought.

Not true. Many of uP adcs have a switched capacitor input which requires a low-impedance driving source.