Errors in an analog to digital converter that cause generated digital codes to deviate from expected values are corrected. The constant current circuit provides a preselected constant current amount positioned between the two sensor states that vary the sensor current signal thereby generating a current level indicator signal. A block substrate having a light receiving surface and a light emitting surface is produced by cutting the composite plate member along a first section parallel to a surface inclining at a predetermined angle with the surface of the composite plate member, the light receiving and light emitting surfaces being parallel to the first section.