US Patent 7971481 Circuit design to prevent false level sense readings

A circuit for conveying signals between one or more ink level sensors and a circuit includes a substrate having a circuit end, a sensor end, and an elongated body extending therebetween. The sensor end includes a bottom layer in which the first signal trace is routed and a top layer in which the second and third signal traces are routed. The sensor end includes a first, a second, and a third connector each extending through the top and the bottom layers that are electrically connected to a first signal trace, a second signal trace, and a third signal trace, respectively, in the substrate. The first connector, the second connector, and the third connector are configured for electrical connection to a first probe, a second probe, and a third probe, respectively, of the level sensor. A first ground trace is positioned in the sensor end that includes a first continuous guard ring in the top layer completely surrounding and spaced from the first connector, and a first segmented guard ring in the bottom layer including a plurality of segments that are each electrically connected to the first continuous guard ring and being spaced from the other segments in the first segmented guard ring. The first signal trace is routed in the bottom layer to the first connector via a space between the segments of the first segmented guard ring.