Digital voltmeter

Claim 1

Apparatus for providing a digital display of a measured analog signal including a periodic source of reference signal in ramp form starting at a negative potential and increasing to a given positive potential, an analog signal to be measured, a first comparator circuit connected to said ramp signal and analog signal and providing a given output level as long as the ramp signal is less than the analog signal, a source of clock signals, first logic means connected to said first comparator and clock source for providing clock signals during the period that said first comparator is at said given level, a second comparator circuit connected to said ramp signal and ground potential and providing a given output level until said ramp signal reaches ground potential, second logic means connected to said second comparator and first logic means for providing clock signals during the period that said ramp signal is above ground potential and less than said analog signal, counting and display means connected to said second logic circuit, input terminals, a rectifier connected to said input terminals, switch means connected to said input terminals, rectifier and first comparator circuit, said rectifier comprising a pair of operational amplifiers, a first diode connected between the output of one amplifier and an input of the other, and a second diode connected between the output and input of said one amplifier.