Dual cavity pressure sensor

A silicon chip (38) mounted on a glass base (34) has two cavities (44, 46) formed therein, each defining a diaphragm (48, 50). Ion implanted resistors (52) are formed in each diaphragm and are connected to a Wheatstone bridge circuit. One of the chip cavities (46) is evacuated to a vacuum reference pressure so that barometric pressure is sensed by the corresponding diaphragm (50) when the atmospheric pressure is applied to the top of the chip (38). The other cavity (44) is connected to a pressure source, such as manifold vacuum of an internal combustion engine so that the corresponding diaphragm (48) will measure the differential pressure between the manifold and atmosphere. The output signals of the bridge circuits are combined in such a way as to give a sensor output proportional to the absolute manifold pressure.