Re: AD conversions

[Nigel Orr <Nigel.Orr@xxxxxxx>]

At 13:32 03/07/00 +0100, you wrote:
>However, the AD converter accepts an input between 0 and 5v, whereas
some of
>the outputs I want to capture run up to 12v. How do I increase the
range of
>the AD converter?

Ignoring source or load impedances (if they cause a problem, you can tweak
the values a bit...), just add a couple of resistors as a potential
divider.  With a 12V signal, you want 7V to drop across one, and 5V across
the one which you read the voltage across.  So any resistors with a 7:5
ratio will do the job- if it's not the sort of thing you have lying around,
something like 1K and 680 ohms should be near enough.

The type of resistor probably isn't critical- power dissipation of 7V
across 1k is 0.05W, and I'm assuming temperature coefficients, noise etc
probably won't make a huge error at these sort of signal levels.  Just pop
down your local Maplin/Tandy/Electrical repair shop and get a handful of
resistors, should cost something between 1p and 10p each.

Connect 12V output - 1K ohm - 680 ohm - 0V and measure the voltage across
the 680 ohm resistor with your ADC.  'Calibrate' it with a 12V source, and
tweak until it is accurate enough.

It's not a beautiful or particularly advanced solution, but it might be
adequate!  If the maximum voltage is more than 12, you could put 3  x 1K
resistors in series, and read the voltage across one of them- that will do
up to 15V, which should be adequate for car things- even batteries on
charge etc.

HTH,

Nigel


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