Patent ID: 7928744

Claim:
A measuring apparatus including a self test function, the circuit comprising: a capacitor; first to fourth switches; a test signal injector; at least one comparator having a signal input and a reference input, the first switch being interposed between a first plate of the capacitor and a first input node, the second switch being interposed between a second plate of the capacitor and a second input node, the third switch being interposed between the first plate of the capacitor and the signal input of the comparator, and the fourth switch being interposed between the second plate of the capacitor and a voltage reference, wherein the self test function comprises the steps of: i) operating the signal injector to produce a first signal representative of an out of range voltage for an expected voltage difference between the first and second input nodes, and using the signal to cause the at least one comparator to place its output in an error state, and to charge the capacitor to the out of range voltage, ii) isolating the capacitor from the signal injector and voltage reference, and connecting the capacitor between the first and second input nodes such that the voltage stored on the capacitor is overwritten by the voltage difference between the first and second nodes, and iii) reconnecting the capacitor to the comparator and monitoring the comparator's output.