Circuit and method for measuring battery voltage by time of discharge of

Abstract

A voltage measurement circuit is provided which can be used in a single supply situation, which has a measurement range from rail to rail and which uses a reference voltage which can lie anywhere between the rails but not at the rail to which measurements are to be referenced. The unknown voltage is sampled to a first plate of a capacitor. The second plate of the capacitor is connected to ground. The first plate of the capacitor is then connected to the first input of a comparator, the second input being connected to receive the reference voltage. If the unknown voltage is less than the reference voltage, the second plate of the capacitor is disconnected from ground and then connected to receive the reference voltage. Otherwise it remains connected to ground. The first plate of the capacitor is then connected to a constant current source, causing the voltage at the first comparator input to decrease linearly with time. By measuring the elapsed time between the connection of the first capacitor plate to the constant current source and a change of state of the comparator due to the voltage decrease at its first input, the difference in voltage between the unknown voltage and the reference voltage or ground can be determined based on the linearity of the time-voltage relationship.