RE: Resistor values Was AD conversions

[Nigel Orr <Nigel.Orr@xxxxxxx>]

At 15:33 03/07/00 +0100, you wrote:
>One final question then, I promise.

OK... I'm only replying as Keith hasn't yet done so, 3 sets of identical
answers in a 24 hour period would be just too worrying...

>This business about noise. Am I best taking the 12/14v signal out of
the
>engine bay uncorrupted, then sampling it somewhere a little
electrically
>quieter?

Do you mean should you put the resistors at the engine end and run the '5V'
signal to the ADC or run the 12V to the ADC and put the resistors there?

I'd be inclined to do the latter, mainly for neatness, but I don't think it
really matters.  Ideally, you want cables that are going through noisy
environments to be carrying high level signals at low impedances, but the
ratio between signal level and impedance will be the same everywhere in a
passive system, so, without actually having written anything down and
calculated it, my guess is "it doesn't really matter" in terms of
noise
pickup.  But I could be wrong...

>  I was planning stuffing the entire circuit on top of the diagnostic
>port since it should work out quite small

But short cables are good, so if the whole thing can go there, that would
probably help.  I say probably as there is one caveat- if the circuitry
around or inside the ADC is at particularly high impedance, it might be
more likely to pick up noise, so you might need to either screen it well or
run a cable to somewhere electrically quieter.

Nigel


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