peter green wrote:
> The problem is for the ladder to work corrrectly you need switches
> that switch between 0V and 5V, not switches that switch between a
> defined voltage level and open circuit.

Are you sure about that?  You are right if you think of the R-2R ladder as
being a voltage source.

Think of it as being a variable resistance instead.  Of course then you nee=
d
a current source to turn the value into a linear voltage, but he's got
enough A/D resolution for a compromise to work.  For example, if a bias
resistor always drops at least 1/2 the voltage, you get at most a 2:1
non-linearity with the readings being spread over half the A/D range.
Another way to look at that is that you lose 2 bits from the A/D result.
That brings a typical PIC 10 bit A/D down to 8 bits in the usable range,
which should be good enough to detect 16 different levels (4 bits).


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