Signal conditioning apparatus and method exhibiting accurate input

Abstract

A signal conditioning apparatus useful in applications, such as automotive applications, having a signal conditioning circuit capable of conditioning a sensor input signal and providing an output useful, for example, to a fuel-air mixture control system. The signal conditioning circuit employs an amplifier with a feedback impedance, NMOS and PMOS transistors, an input impedance, a detection impedance, input terminals coupled to a sensor. The sensor, such as an oxygen sensor, has a sensor output referenced to sensor ground and input terminals coupled to a power supply and power supply ground. The signal conditioning circuit utilizes switched impedances to provide an output voltage proportional to the differential voltage between the sensor output voltage and the sensor ground when an impedance between the signal conditioning circuit input terminals is low relative to the detection impedance. Otherwise the signal conditioning circuit utilizing switched impedances provides an output voltage proportional to the power supply voltage. The signal conditioning circuit samples the sensor differential voltage and the power supply voltage at a frequency derived from a clock generator. The signal conditioning circuit controls the NMOS transistors coupled to the sensor with a negative bias voltage derived from a single power supply, thus allowing operation when the sensor ground and power supply ground are at different potentials. Additionally, fabricating the signal conditioning circuit impedances as capacitors using linear complimentary metal oxide semiconductor technology produces very accurate gain and input impedance characteristics.