Patent ID: 7416650

Claim:
A gas concentration measuring apparatus comprising: a gas sensor configured to measure a concentration of a specified gas component contained in a gas and to output a sensor current corresponding to the measured concentration of the specified gas component; and a measurement substrate where an electric circuit is formed, said electric circuit being electrically connected to the gas sensor and including a signal processing circuit configured to measure the sensor current outputted from the gas sensor, wherein said electric circuit comprises: a connection terminal electrically connected to the gas sensor and configured to receive the sensor current from the gas sensor, said connection terminal having input impedance of 500 kΩ or over; a conductive pattern portion having conductivity and formed in the measurement substrate; and an electric component mounted on the conductive pattern portion, said conductive pattern portion including: a signal input pattern electrically connected to the connection terminal, said signal input pattern having direct current impedance with respect to the connection terminal, said direct current impedance being 10 percent or less of the input impedance of the connection terminal; a different potential pattern having a potential difference of 2 V or over from a potential of the signal input pattern; and a guard pattern having a substantially constant potential and a potential difference of less than 0.5 V from the potential of the signal input pattern, said guard pattern being arranged on at least a portion of the measurement substrate, said at least portion of the measurement substrate being located between the signal input pattern and the different potential pattern, wherein the conductive pattern portion includes a signal measurement pattern constituting the signal processing circuit, a potential of the signal measurement pattern depends on that of the signal input pattern, the signal processing circuit comprises an operational amplifier, the signal input pattern is connected to a non-reverse input terminal of the operational amplifier so that the potential of the signal input pattern is input to the operational amplifier via the non-reverse input terminal thereof, an output terminal of the operational amplifier is connected to a reverse input terminal thereof so that the operational amplifier is configured to output, via the output terminal, a voltage that substantially equals to the potential of the signal input pattern, and the guard pattern is electrically connected to the signal measurement pattern.