RE: AD conversions

[Nigel Orr <Nigel.Orr@xxxxxxx>]

At 14:15 03/07/00 +0100, you wrote:
>Cheers Nigel and Keith!

Glad we both came up with the same figures- even if I went the long way
round...

>Do I need to worry about the fact I'm effectively connecting the output
from
>the ECU to ground via 3 1k resistors?

Maybe.  It depends... some ECUs have odd 'modes' eg for service etc which
are usually entered by shorting something to something else, usually on the
input side though.  If that isn't an issue, then there's just it's load
driving capability to worry about, and that _shouldn't_ be a problem, 12V
into 3 kohm is 4mA, which is a fairly trivial load, and unlikely to be a
problem if it is an analogue output.  If it's digital, CMOS outputs, and
not buffered (which is unlikely in an automotive'y thing, but possible), it
might have a maximum output current under 1mA, so you could start with 10k
resistors, and use them if there are no problems.

If you have a voltmeter, you could connect 3V across the output and see if
it affects it at all.  If it doesn't change markedly, you're probably
fine.  Maybe the ECU manufacturer would have useful data on it's drive
capabilities if you ask nicely.

> high impedance(?), but does 3kohm count as high impedance and hence
safe?

I think of 1k as 'middle' impedance... and probably Keith does too, which
is why we both went for that...  Under 100 ohms, might draw too much
current and upset the source, over 10k, the load impedance might start to
be an issue.  It all depends on what the source and load actually are, so
1k is usually a good place to start...

Nigel


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