Classically an R2R ladder is used to implement a Successive
approximation Analog to Digital converter with drives being high or
low. It seems to me that a ternary converter using
high/low/open-circuit outputs should allow more resolution per pins
used (by a factor of up to 3^N/2^N).

Does anyone know if this is commonly done and if so, what resistor
combinations are usually used?
Initial playing on paper produced some ideas (I should be able to get
"8 bit" accuracy with 6 pins used) but I may be missing something
obvious. Theory says 5 pins SHOULD allow 1 part in 243 but obvious
implementation escapes me at this stage.



Russell McMahon