RE:Resistor values Was AD conversions

[Keith Doxey <keith.doxey@xxxxxxx>]

Spookey  :-))

Nigel must have the same stock of resistors as me :-)
1K and 10k in quantities of 1000's and a handful of other values in 10's or
20's.

I also assumed that because of the noisy (electricaly) environment the
driving signal would almost certainly be quite low impedence and kept the
resistor values reasonably low to minimise the possibility of noise pickup
on the divider network. Also I took the A/D input to be >10k or even
100k.

At college we were told that for rough calculations anything an order of
magnitude out could be more or less ignored. ie. 10k in parallel with 1k
still roughly equals 1k rather than 910 ohms.

Good to see that our second responses were almost identical too.
It means either we both have it right or have made the same mistakes :-)

Keith

> -----Original Message-----
> From: Nigel Orr [mailto:Nigel.Orr@xxxxxxx]
> Sent: 03 July 2000 14:27
> To: ukha_d@xxxxxxx
> Subject: RE: [ukha_d] AD conversions
>
>
> 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...
>
>  > 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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