Patent ID: 7671600

Claim:
A gas concentration detection apparatus comprising a sensor element which incorporates a solid electrolyte layer and is responsive to an applied DC voltage for passing a concentration-indicative current varying in level in accordance with of a concentration of a specific constituent gas in a test-object gas, an AC voltage source and a DC voltage source for respectively applying an AC voltage and said DC voltage to said sensor element, a current measurement resistor connected in series with said sensor element in an AC current path through which an AC current component flows in response to said applied AC voltage, and a detection signal output circuit for deriving a gas concentration detection signal indicative of said concentration of the constituent gas based on said concentration-indicative current and deriving an impedance detection signal indicative of an impedance value of said sensor element based on said AC current component; wherein said detection signal output circuit comprises: a first detection signal output circuit for deriving said gas concentration detection signal, coupled to said current measurement resistor for extracting a first voltage signal, with said first voltage signal varying in level in accordance with said level of the concentration-indicative current, and a second detection signal output circuit for deriving said impedance detection signal, coupled to a connection point between said current measurement resistor and said sensor element for extracting a second voltage signal, which said second voltage signal being an AC signal which varies in amplitude with respect to a circuit ground potential in accordance with an amplitude of said AC current component.