The present invention provides a reduced wire count voltage-drop sense
system and method wherein the voltage drop across a load device is
determinable using only one sense lead regardless of the number of load
devices. In one embodiment, the voltage drop across any one of four LEDs
(220A-D) in a photoplethysmographic probe when a drive current is applied
therethrough via one of four input drive leads (230A-D) connected to
separate LED terminals (222A-D) is determinable from a first voltage drop
and a second voltage drop, for example, by subtracting the second voltage
drop, or a portion thereof, from the first voltage drop. The first voltage
drop is measurable across a terminal (232A-D) of the input drive lead
(230A-D) connected to the LED (220A-D) across which the voltage drop is to
be determined and a terminal (252) of a sense lead (250) connected to a
common LED terminal (224). The second voltage drop is measurable across
the sense lead terminal (252) and a terminal (242) of a common drive lead
(240) connected to the common LED terminal (224).
