Diagnostic circuit and method for amperometrically determining the current passing through a sensor

Claim 1

Diagnostic circuit for amperometrically determining the current passing through a sensor (41), comprising a current source (V+, 46) for passing a constant DC current through said sensor (41), switching means (45) for selectively operating said current source, comparing means (42) for comparing the potential difference (V BS ) across said sensor to a reference voltage (V ref ), and producing a control signal (D) having a first logic level when said potential difference is less than said reference voltage (V ref ) and a second logic level when said potential difference is greater than said reference voltage (V ref ), control means (44) for providing a switching signal (Q) having said first logic level in response to said control signal (D) having said first logic level and said second logic level in response to said control signal (D) having said second logic level, said switching means (45) being caused to operate when said switching signal (Q) is in said first logic level, and timing means (48,49) for measuring the cumulative time during which said switching signal (Q) has said first logic level during a selected time interval, so as to provide an output signal (50) indicative of a mean value of current (I bs ave) passing through said sensor (41).