On Sun, 13 Jun 2010 18:33:12 -0600, "Forrest W Christian" said:

> 
> In an ideal world, I'd just do a 100K and 5.6K... unfortunately the 
> 'minimum' input impedance is 10K on the 'f886.

I think you mean "maximum"??

Remember your Thevenin equivalents - the source impedance will be the
same as the parallel resistance of the two resistors, in this case just
over 5K.

So you don't have a problem, if the 10k figure is right, I don't have
the datasheet in front of me, and the value is different on different
parts..

> 
> I would prefer to avoid adding a opamp or other buffer.  I remember some 
> discussion that you can actually get away with violating the input 
> impedance spec, especially if you add a input capacitor.  Since this is 
> a DC source, can I just do the 100K and 5.6K and say put a .1uF cap on 
> the input pin?  What am I traiding off.  Other ideas?

Having too-high source impedance does two things that hurt the accuracy.
First, the sampling capacitor needs to charge from the source, second is
input bias current. By adding a cap you fix the problem of the sampling
capacitor but it doesn't fix the input bias current, so your error is
usually skewed toward mid-scale.

Cheerful regards,

Bob


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